Deadly CO2 gases in the Plutonium of Hierapolis (Denizli, Turkey)

  • Hardy Pfanz
  • Galip Yüce
  • Ahmet H. Gulbay
  • Ali Gokgoz
Original Paper


Using a portable gas analyzer system, the geogenic gas regime below and around an ancient gate to hell at Hierapolis/Phrygia was characterized. The site was first described by Strabo and Plinius as a gate to the underworld. During centuries, it attracted even ancient tourists. In a grotto below the temple of Pluto, CO2 was found to be at deadly concentrations of up to 91%. Astonishingly, these vapors are still emitted in concentrations that nowadays kill insects, birds, and mammals. The concentrations of CO2 escaping from the mouth of the grotto to the outside atmosphere are still in the range of 4–53% CO2 depending on the height above ground level. They reach concentrations during the night that would easily kill even a human being within a minute. These emissions are thought to reflect the Hadean breath and/or the breath of the hellhound Kerberos guarding the entrance to hell. The origin of the geogenic CO2 is the still active seismic structure that crosses the old town of ancient Hierapolis as part of the Babadag fracture zone. Our measurements confirm the presence of geogenic CO2 in concentrations that explain ancient stories of killed bulls, rams, and songbirds during religious ceremonies. They also strongly corroborate that at least in the case of Hierapolis, ancient writers like Strabo or Plinius described a mystic phenomenon very exactly without much exaggeration. Two thousand years ago, only supernatural forces could explain these phenomena from Hadean depths whereas nowadays, modern techniques hint to the well-known phenomenon of geogenic CO2 degassing having mantle components with relatively higher helium and radon concentrations.


Carbon dioxide Helium Radon Charonion Gate to hell Geogenic gases Hades Mephitic exhalations Mofette 


The Pamukkale thermal province located in the east of the Buyuk Menderes Graben is famous for its thermal springs and snow-white travertine formations and also for the excavations of ancient Hierapolis. First recordings of the old city of Hierapolis were made by Strabo (XII, 8, 17) and also Plinius the Elder (Nat. Hist. V, 105) mentions this famous town. The town, probably a Seleucid foundation in the second century B.C. (Gerster 2005 p. 162; Porter 2016 p. 355), developed during the Roman Empire and was already famous in Byzantine times (Ring et al. 1995 p. 327; Rigsby 1996; Cohen 2006; Zwingmann 2012; Nyquist 2014; Şimşek and D’Andrea 2017). In this period, a huge pilgrim sanctuary was built around the tomb of the Apostle Philip (D’Andria 2003). Geologically, the holy town Hierapolis is cut longitudinally by several parallel fractures of the Pamukkale fault (intra-plate tectonics) as the part of extensional straining of the western Anatolian extensional regime or the west Anatolian horst-graben system extending from the Aegean Sea to central Anatolia (Seyitoglu and Scott 1996; Kaymakci 2006). It is specified mainly by major horsts and grabens at E–W trending, and NW–SE to NE–SW oriented relatively short and locally suspended cross-grabens (Bozkurt 2003). Thus, Hierapolis is situated in one of the most tectonically active regions of Asia Minor. Therefore, the area was destroyed several times by many earthquakes (Altunel and Barka 1996; Hancock and Altunel 1997; Altunel and Karabacak 2005; Kaymakci 2006; Uysal et al. 2007, 2009; Kele et al. 2011; Hancer 2013; De Filippis et al. 2012; Kumsar et al. 2016). Directly build upon such a fault are two interesting buildings: the famous Apollo temple and the newly discovered Plutonium, the sanctuary of the Gods of the Underworld, Pluto and Kore, with the theater above a grotto. The latter was excavated between 2011 and 2013 during the campaign of the Italian Archaeological Mission in Hierapolis (D’Andria 2013). To be exact, this site has been mentioned by several antique sources as the entrance to the Underworld (Zwingmann 2012); they describe that strange things happened at this Hadean outlet which they called Plutonium (Ploutonium) or Charonium. Deadly vapors were described escaping from these places (Strabo XIII, 4, 14; Bejor 1984). Priests were demonstrating their supernatural power and their equality to the gods by ushering animals like goats and bulls into the grotto (Plutonium) where, after a short time, the animals showed signs of suffocation, finally dying after several minutes. Yet, the castrated priests (Galli) survived (Strabo XIII, 4, 14; Plinius, Nat. Hist. II, 207–208; for reviews, see Zwingmann 2012 or Pfanz et al. 2014).

Many similar sacred Greek and Roman places, oracles, and temples were often located directly above or close to geological disturbance zones (Piccardi 2000; De Boer et al. 2001; Etiope et al. 2006; Foster and Lehoux 2007; Piccardi et al. 2008; Mariolakos et al. 2010). The famous oracle of Delphi was built on the intersection of two fracture zones where either methane/ethylene venting or hypoxic aeration due to methane or CO2 emission enabled Pythia to give her prophecies (De Boer et al. 2001; Etiope et al. 2006; Piccardi et al. 2008). Heavy methane emissions were also found at Chimera (Etiope et al. 2006; Hosgormez et al. 2008). In all cases, either high CO2 concentrations or methane and other hydrocarbons were emitted from geogenic sources (for a review, see Pfanz et al. 2014 and Etiope 2015).

This paper accounts for the geochemical compositions of soil gas of some fractures and holes around the Hierapolis fault and Hierapolis archeological sites. We concentrated our survey specifically on the close vicinity of the Temple of Apollo and the Plutonium of ancient Hierapolis/Phrygia where mystic tales and narratives of the asphyxiation of bulls during religious ceremonies exist. We tried to (1) find and (2) quantify the noxious gas(es) inside and outside of the both temples and (3) examine the diurnal degassing patterns to help understanding how ancient ceremonies were safely performed by the priest of the sanctuaries.

Material and methods

The location

Hierapolis is located in the Denizli graben in the southwest of Turkey (Fig. 1a, b) which is a geological disturbance zone extending between the Pamukkale and Babadag faults that are paralleling each other (cf. Özdemir 2002; Kaymakci 2006; Hancer 2013; Kumsar et al. 2016). Several buildings of Hierapolis were directly built on the fracture zone. Two buildings that were already mentioned by ancient writers for their “deadly Hadean breath” were selected for a close determination of geogenic gas emissions: the Apollo temple and the newly excavated sanctuary of Pluto (Fig. 1c; see also D’Andria 2013).
Fig. 1

Location of the area of Hierapolis/Denizli in southwest Turkey (a). Position of the two sanctuaries, the sanctuary of Pluto (Plutonium) and the Temple of Apollo (top left) in the centrum of the Hierapolis excavated area. The positions of sinkholes in the close vicinity of the sanctuaries are given (b). Known faults and grabens in the Denizli graben and its rims including the towns of Hierapolis, Laodikea, and Denizli. Maps were modified from Kumsar et al. 2016 (c)

Gas measurements

Measurements were taken during two different campaigns in May 2013 and June 2014.The gas measurements were carried out with the portable gas analyzer GA2000 (Geotechnical Instruments, England; Pfanz et al. 2004) equipped with water and dust filters. CO2, CO, CH4, H2S, and O2 were recorded simultaneously. Problems occurred when the soil was wet, waterlogged, or filled with hot water vapor, as liquid water would damage the sensitive cells of the analyzer. Polyethylene and Teflon tubing (1 cm in diameter) was used to enlarge the measuring radius up to 6 m. Readings were recorded after several seconds to one minute. Long-term measurements for CO2 were made using a self-made CO2 analyzer, equipped with an ultra-low power (3.5 mW), non-dispersive infrared (NDIR) sensor (measurement range of 0–100%; COZIR, CO2 Meter, Florida). The warm-up time of the sensor is shorter than 10 s; operating conditions are − 25 to 55 °C and 0–95% RH, non-condensing. The accuracy of the sensor is ± 0.5% vol. CO2, at standard temperature and pressure. The pressure dependence of the sensor is 0.13% of the readings per millimeter of Hg.

The sensor was attached with a micro-diaphragm gas sample pump (delivery, 0.4 l/min; KNF Neuberger, Balterswil, Switzerland). The sampling time was set with a microcomputer to 30 s. At the end of each interval, the measurement value was saved on a micro-SD card and the sensor and pump were programmed to go to sleep mode until the next sampling cycle (after 5 min). The battery-driven device was mounted within a water-tight hard plastic box. CO2 measurements at different heights (15, 40, 100 cm) above the ground level within the sanctuary of Pluto were performed on different days in 2014.

In situ radon measurements were carried out by GEO-RTM 2128 alpha spectroscopy instrument. A Teflon tubing was inserted 2–3 m into the mouth hole of the Apollo temple and grotto of the Plutonium; it was directly connected with the radon measurement chamber. The relative statistical error for radon activities ranged from over 10% for low radon values (∼ 2–10 Bq l−1) to less than 5% for values greater than 10 Bq l−1. The measurement period was selected as a minimum of 1 h to obtain reliable results with a high confidence level (95%). The isotopic composition of helium and neon was determined by mass spectrometry at INGV-Palermo. Two gas samples taken directly from the mouth of the grotto were collected by taking care to avoid the least amount of atmospheric gas contamination. The sampling procedure for free gases is as follows: free gases were collected into the pyrex bottle having a three-way valve connected to a silicon tube which was inserted into the grotto. The silicon tube was then connected to a syringe which sucked the gas through the valve directly into the sampling pyrex bottle. To ensure pure geogenic gas, the bottle was flushed with a gas volume ten times larger than its own volume. For the final gas collection, the first valve was closed and by applying a slight overpressure insight the bottle; the second valve was finally closed (Italiano 2009 and Italiano et al. 2013).

Chemical analyses were done by gas chromatography (Perkin Elmer Clarus 500 equipped with a double TCD-FID detector) using argon as the carrier gas. Uncertainties were within ± 5% (Italiano et al. 2013). The isotopic compositions of helium were obtained by mass spectrometry. The isotopic analyses of the purified helium fractions were performed by a split flight tube static vacuum mass spectrometer (GVI5400TFT) that allows the simultaneous detection of 3He and 4He-ion beams, thereby keeping the 3He/4He error of measurement to very low values. Air is routinely run as a standard for calibration. In general, the total errors on the ratios are less than 2 and 5% in one sigma standard deviation, respectively (for 3He/4He and 4He/20Ne). Uncertainties in the range of low 3He (R/Ra values below 0.1) samples are within ± 5% (Italiano et al. 2013).

The 3He/4He ratios (R) have been normalized to the atmosphere (3He/4He = 1.39 × 10−6 = Ra) and corrected for the effects of atmospheric contamination (R/Rac) using (R/Rac) = {(R/Ra) X − 1} / {X − 1} where X is the air and the ASW He/Ne ratio (Hilton et al. 1998).

Local weather data were obtained from the Turkish State Meteorological Service (DMIGM) for Denizli.


The new Plutonium—a deadly gas atmosphere around and within the sanctuary of Pluto and Kore

During the excavation held by D’Andria in 2011–2013, a subterranean grotto was found below the stone-seats of the Plutonium (Fig. 2a). The grotto belongs to the sanctuary of Pluto (Hades) and Kore (Persephone). The words “Ploutoni kai Kore” in Greek letters engraved into the stone row below the seats are still readable (D’Andria 2013). Due to the incomplete excavation state of the Sanctuary in 2013, only the small, upper part of the grotto and the antechamber was accessible for gas measurements (Fig. 2a). In 2013, access to the interior of the subterranean grotto was possible only through a small mouth hole (red arrow in Fig. 2a). One year later (in 2014), the lower part of the Plutonium was fully excavated and freed the old basement (Fig. 2b). Unfortunately, hot, carbonate-rich water poured into this basement from the interior of the grotto. The whole front area below the seat rows was thus flooded with water up to a height of 10 cm. Thus, proper CO2 gas measurements were only possible above the water phase (ca. 15 cm aboveground).
Fig. 2

Stone seating rows for the spectators of the ceremonies as seen in the freshly excavated Plutonium within the Sanctuary of Pluto, picture from 2013. The antechamber in front (blue arrow) of the subterranean grotto (white arrow) is seen. Also shown is the hole within the walls of the grotto where the geogenic gas escaped from the interior to the atmosphere (red arrow) (a). A similar scene of the Plutonium made in 2014 when excavations had finished and the proper basement was fully freed (b)

Gas data from the 2013 campaign

Due to the incomplete excavation state of the Plutonium in 2013, gas measurements inside the closed subterranean chamber (grotto) of the Plutonium were performed by inserting a 6-m long Teflon tubing into the small opening of the excavated wall (arrow in Fig. 2a). At a depth of 2 m, maximum CO2 concentrations inside the grotto were 91%. Measurements were herewith done at the mystical place where hellhound Kerberos is expected to have its guard post (Bloomfield 1904). Yet, due to the small mouth hole and the total darkness inside the grotto, the exact position of the end of the tubing could not exactly be located. The complete basement of the grotto was totally dark but seemed to be warm and highly humid (due to a warm carbonate-rich creek).

All CO2 measurements within the grotto gave similar results, independent of the exact position of the tubing. A rather homogeneous gas lake was therefore assumed within the grotto with more or less constant CO2 concentrations ranging between 86 and 91%. As neither wind nor sun could enter the closed grotto, no large changes of the gas concentrations were to be expected. Only close to the small wall opening, CO2 concentrations slightly decreased. Due to some mixing with the normal atmosphere outside, around 74% CO2 were seen. Due to the extreme CO2 concentrations within the grotto, oxygen concentrations were well below ambient; only 6–2% O2 were found within the grotto, the rest being nitrogen (N2).

The situation was totally different for the sanctuary’s floor, the antechamber outside the grotto (Fig. 2a; blue arrow). Surrounded by walls, which prevented the rapid discharge of the gas, the floor was continuously flooded with CO2 during the whole day. As CO2 is 1.5 times heavier than air, the escaping CO2 gas forms a gas lake at the floor (Pfanz et al. 2014, Kies et al. 2015). Depending on atmospheric and microclimatic conditions, the CO2 gas lake on the floor of the antechamber varies diurnally. It has again to be mentioned that due to the incomplete excavation state of the Plutonium in 2013, the location of the floor does not represent the original proper floor level (but see Fig. 2b). Yet, even at this higher level of the floor, a small gas lake existed. It has to be stressed that a deadly CO2 gas lake existed only under certain weather conditions: no direct sunlight and no strong air movement. On sunny or windy days, no gas lake was measurable during daytime but occurred through the night.

Yet, measurements of the gas lake performed during early morning hours clearly proved the presence of CO2. Although it was mostly slightly windy during our first campaign, the actual CO2 gas lake had a maximum height of 35 cm above the floor of the antechamber with CO2 concentrations reaching 57% at the very bottom. As expected, CO2 concentrations dropped with height due to wind dilution. At 10 cm aboveground, CO2 was reduced to 36% and further declined via 9.0 and 3.9% CO2 at a height of 20 or 30 cm, respectively. At 40 cm above the ground level, measured CO2 concentrations were close to ambient (0.04–0.05% CO2). At the same time, oxygen values increased from 8.0–9.3% at the very bottom via 12.7 and 18.7 to finally reach 19.3–20.7% at 40 cm height. During all morning hours, observed CO2 concentrations at the very bottom of the antechamber of the Plutonium were always in the range between 27 and 76%. At the same time, oxygen values were very low (6.6–12.5%). The reason for the broad concentration range of both gases was the fluctuating wind and solar irradiation.

In addition, the great number of corpses of insects and birds corroborated the existence of a deadly CO2 gas lake in front of the grotto. On our first day, two dead birds and more than 70 dead beetles (Tenebrionidae, Carabaeidae, and Scarabaeidae) were found asphyxiated at the floor (Fig. 3). Locals report on dead mice, cats, weasels, and even asphyxiated foxes. Most animals are not killed during sunny days, but during the dark evening and morning hours. For small organisms like insects, there is a danger to get asphyxiated even during daytime; they were seen to be asphyxiated even during midday hours. The reason for this is the persistent thin gaseous CO2 coat that covers the lowest 5 cm of the very bottom throughout the day.
Fig. 3

Gas victims in front of the grotto. The beetles were killed during the high-CO2 phase of the gas lake persisting through the dark hours. As a very thin gas lake (up to 5 cm in height) also exists during daytime, insects also suffocate during sunny hours. Dead beetles were from several families: Tenebrionidae, Carabaeidae, and Scarabaeidae

Data from the 2014 campaign

Due to the advanced excavation state of the lower part of the sanctuary (Fig. 2b), diurnal gas fluctuations within the antechamber of the Plutonium could be measured continuously in 2014 (Table 1). As the floor was flooded with water from a thermal creek, the gas analyzer was positioned at three different heights above the floor of the antechamber: (i) directly above the water level (15 cm; on stones which emerged from the water table) or (ii) 40 cm or (iii) 100 cm aboveground. The data given in Fig. 4 show the differences of the diurnal CO2 gas regimes at these heights. Varying the height of the measurements, the potential threat of the deadly gas to different organisms could be simulated. Furthermore, continuous measurements exactly proofed at which time of the day the highest danger was to be expected. As only one CO2 device was available, the data at different heights represent different days (Fig. 4).
Table 1.

CO2 and radon concentrations as measured at selected locations in the vicinity of two Sanctuaries. Measurements were performed in 2014







Rn (kBq/m3)

CO2 (%)































































































































*in air

**in waterFF-9

Fig. 4

Diurnal courses of the CO2 gas concentrations in front of the antechamber of the grotto within the sanctuary of Pluto. Measurements were performed for 24 h (from 09.00 p.m. to 09.00 p.m.) at three different heights aboveground (lowest level 15 cm; medium level 40 cm; and upper level 100 cm). Measurements at the proper ground level were not possible as the base of the antechamber was flooded with water draining from the hot carbonate creek running downhill through the grotto. The weather at these days was quite similar. Rather hot and sunny during daytime (temperatures up to 38 °C) and strong rain events and thunderstorms in the afternoon or during the night. As only one CO2 device was available, the data at different heights represent different days. Average temperature and wind speed are given. Rainfall events are marked in red. Rainfall and wind speed values are exaggerated by a factor 10

When the CO2 analyzer was mounted 100 cm above the ground of the antechamber, nearly no excess concentrations of CO2 were found during a diurnal cycle (Fig. 4). Measured data ranged between 0 and 5% CO2. Nevertheless, it has to be mentioned that CO2 concentrations around 5% can lead to dizziness in humans if inhaled for longer than 5 min (IVHHN 2013). The picture was clearly different when the CO2 monitor was placed closer to the ground (40 cm aboveground). A dramatic increase of CO2 was now seen during the dark hours. Depending on the prevailing wind and rain conditions, CO2 ranged between 2 and 36% (Fig. 4). Yet, in most time of the day, concentrations were below 10% CO2. Those concentrations will asphyxiate and kill humans and other mammals within minutes. Even higher CO2 concentrations were reached with the monitor placed directly above the water phase of the antechamber’s floor (15 cm above the proper floor). During the, night extremely high CO2 were found (Fig. 4). The diurnal CO2 concentrations were neither constant nor were they stable. But even during daytime, concentrations were rather high (5–20%) but they still increased in the evening hours to reach maximum levels during late evening and night (Fig. 4). Deadly 68% CO2 were then measured in the atmosphere close to the bottom (sometimes even 85% CO2; not shown). During morning hours, these concentrations were quickly reduced due to solar irradiation (see also Kies et al. 2015).

Radon concentrations in both temples (9 and 55 kBq m−3) can be considered as higher than normal since they are located in the same active fracture zone. Radon concentrations mostly paralleled the concentrations of geogenic CO2 (Table 1). This behavior is understandable as radon is transported from its geological source within a matrix of CO2 as the carrier. It becomes clear that geogenic gas emissions frequently occur within the fracture zones of the graben structure cutting its way through ancient Hierapolis.

Aside from carbon dioxide, other gases could also be measured within the grotto, but only in very low concentrations. Carbon monoxide was present in minute concentrations (1–3 ppm) and oxygen concentrations were down to 3.2%. Methane and hydrogen sulfide were absent, whereas radon concentrations were found to be between 50 and 55 kBq m−3 in both places. The chemical analysis (Table 2) is also consistent with the in situ measurements. The air corrected helium isotope ratios of the two samples are 3.61 and 3. 78 Rac, respectively. These values are consistent with a previous work of Ercan et al. 1995 (air corrected R/Rac = 3.68) and Gulec et al. 2002. The concentration of helium is above the atmospheric value and CO2 is the dominant gas possibly carrying mantle sourced helium. The relationship between 3He/4He and 4He/20Ne ratios shows mixing curves between atmospheric and mantle/crustal components (Fig. 8) assuming that an atmospheric component has 3He/4He = 1.39 × 10−6 and 4He/20Ne = 0.318, and a crustal component has 0.02 Ra and 4He/20Ne = 1000 (Sano and Marty 1995). The presence of different mantle-type components is also considered: MORB-type mantle with 8 Ra and 4He/20Ne = 1000, sub-vontinental European mantle (SCEM, Dunai and Baur 1995) R/Ra = 6.5 and 4He/20Ne = 1000. The mantle contribution was calculated about 75% (Table 2) and the radon content in the gas samples confirmed deep origin with remarkably higher values. It is obvious that this higher helium isotope ratio with an elevated CO2 and radon content strongly hint to the tectonic activity and proves the ascent of mantle volatiles to the surface. Moreover, the N2/O2 ratio is well above that of atmospheric ratio (~ 2) which is consistent with the higher helium isotope ratio (Table 2).
Table 2

Gas analysis of the Pamukkale spring as determined by the Noble Gas Laboratories of INGV, Palermo

Sample ID









[He] Corr

[Ne] Corr


Mantle Contribution Rate

% Atm

% Rad

% Mag

Pamukkale 1















Pamukkale 2















The old Plutonium—geogenic gas below the temple of Apollo

Similar findings were obtained when the grotto below the temple of Apollo was studied. The sanctuary of Apollo is situated 200 m west of the sanctuary of Pluto within the same seismic fracture zone due to the extension of the fault line and it is long known for its subterranean grotto-like structure (Figs. 1 and 5). This cellar-like chamber below the Apollo Sanctuary is marked as “Plutonium” in touristic guides of Hierapolis (D’Andria 2003). The toxicity of its gas atmosphere is known and the entrance to the cellar below the temple was therefore firmly walled due to safety reasons.
Fig. 5

Radon measuring set in front of the closed chamber of the Apollo temple. The CO2-filled room below the temple has been sealed by walls. Gas measurements were therefore made by inserting the tubing through the small window that was left open

When the Teflon-tubing was inserted 2–3 m into the mouth hole of this grotto, between 60 and 65% CO2 were to be measured independent of the exact location of the tubing. At the same time, oxygen was around 8–8.5%. Also, here, like in the neighboring new Plutonium, a deadly geogenic CO2 atmosphere accumulates within a walled chamber. And again, this gas emission reflects the breath of the deadly underworld in ancient times. Because the originally existing larger opening had been sealed due to safety considerations, no enhancement of CO2 concentrations could be observed outside the hole.

Geogenic gas in the closer surroundings of the two sanctuaries

Since both sanctuaries are located in the same seismic fracture zone of the Hierapolis fault (De Filippis et al. 2012). Gas analyses were enlarged to the close vicinity of the temples. Therefore, some measurements were performed within the linear fracture zone east and west of the two sanctuaries (Fig. 6a). Several CO2-emitting spots were found in sinkholes in the vicinity of the sanctuaries (Fig. 6b). In the south of Pluto’s sanctuary along the graben lineament, gas concentrations of up to 84% CO2 were measured in small crevices. Increased CO2 was also found in the seismic graben structure itself but also in sinkholes around the fracture zone. Yet, the highest CO2 concentrations were found within the grotto of the sanctuary of Pluto and at a sinkhole some 25 m east of the sanctuary. Geogenic CO2 decreased in southeast and northwest directions, although concentrations were still deadly until locations F3 (141 m) and F15 (92 m; Fig. 7a, b). The fractures and sinkholes in the close vicinity of the two sanctuaries are not evenly distributed; therefore, the sites for gas measurements had to be adapted to the local settings. Since the survey was focused on the closer vicinity of both temples, more distant natural springs and fractures were excluded.
Fig. 6

Fracture zone thorough a travertinic rock east of the sanctuaries (a). Gas tubing was inserted into a sinkhole in the vicinity of the sanctuaries (b)

Fig. 7

Location of the different measuring points of CO2 and radon contents within the settings of the two sanctuaries of Apollo and Pluto. The points were selected according to the morphology of the site and the direction of the fissure (a). CO2 and radon concentrations as measured at the locations given (b). Data from both campaigns in 2013 and 2014. CO2 concentrations are given in percent; radon is given as kBeq/l.


Physical and meteorological behavior of geogenic CO2

Within the two sanctuaries of Apollo and Pluto, geogenic CO2 was found in subterranean chambers (D’Andria 2013). These grottos were situated either below the proper temple (in the case of the Apollo temple) or below the seating rows for the pilgrims and spectators (in the case of the sanctuary of Pluto). In both cases, more than 60% CO2 accumulated within the walled grottos as neither sunlight nor wind was able to dilute this poisonous atmosphere.

The gas that permeated through cracks and holes to the outside of the grotto of the sanctuary of Pluto showed a different behavior. Due to the higher density of CO2, the diurnal buildup of a CO2 gas lake occurred on the very bottom in front of the grotto.

CO2 concentrations in the antechamber of the grotto located directly ahead of the sitting rows for spectators and pilgrims were highest close to the bottom (Fig. 4). With increasing height, CO2 concentrations decreased. Furthermore, sunlight, temperature, and wind induced a diurnal pattern in the gas lake of the antechamber.

Within the antechamber, the formation of a higher-concentrated CO2 gas lake occurred only during the dark hours of calm nights. Under windy conditions or during the sunny hours of a normal day, the gas lake was partially or wholly destroyed due to the strong infrared absorption of CO2 and the concomitant heating of the system (Pfanz et al. 2004, 2014). The heating of the gas lake leads to a decrease of its density and to its disintegration and final dissipation within the above-lying air parcels. Similar observations have been made for a larger, natural CO2 gas lake in Italy (Kies et al. 2015). Furthermore, a closer look to Fig. 4 shows that even within this diurnal, sinus-like “CO2 wave” of the gas lake, concentrations were not ideally stable for a longer period. A heavy scatter of CO2 concentrations mainly during night hours is seen (Fig. 4). Concentrations varied from 68 to 10% CO2 within minutes. The reason for these sudden CO2 changes were wind gusts and heavy rain events occurring during our measurement campaign (see also Bettarini et al. 1999; Etiope et al. 2005; Pfanz 2008; Kies et al. 2015). Meteorological data compiled from the region are well consistent with the CO2 data. There is an inverse relationship between the CO2 gas concentration and air temperature, rainfall, and wind causing a CO2 decrease (Fig. 4).

Gate to hell—the historical relevance of the gas findings

Biological effects of extreme CO2 concentrations

Several old writers report on the death of birds, rams, and bulls in the vicinity of the two Plutonia. The measured data clearly proof that even nowadays, CO2 concentrations and the concomitant low oxygen concentrations within the grottos and antechambers of the sanctuaries of Pluto and Apollo are highly toxic. So clearly, oxygen-depending life will not exist within the grotto of Plutonium. Neither mammals nor reptiles or birds can survive. Even insects are killed within minutes (for some exceptions, see Russell et al. 2011). Outside the grottos, in the antechambers, the situation is different. Depending on the size/height of the respective animal (and depending on the time of the day and the weather condition), the situation could be either deadly toxic or only slightly dangerous. Mammals already react to CO2 concentrations as low as 3–5%. Even these rather low concentrations may increase cardiac frequency and respiratory rate if incubation time is longer than several minutes (D’Alessandro 2006; Pfanz 2008). At CO2, concentrations around 8–10% humans are asphyxiated and a longer incubation at 15–20% inevitably leads to death (IVHHN 2013).

Historical relevance of the gas findings

Entrance to hell

To the ancient people, these gas-emitting places were extremely frightening and therefore holy and consecrated. Such sites were often found just accidentally by herdsmen or farmers watching their cattle behaving strange. Herdsmen were astute in observing nature; even a slight change or variation in the normal vegetation (Pfanz et al. 2004) could alert them to such extraordinary places (Pfanz et al. 2014). Often temples and sanctuaries were then erected at these sites. These sacred sites resembled the entrance to the Underworld, the antechambers of the Hadean. Deadly vapors were escaping from these outlets (Strabo XIII, 4, 14). The toxic vapors resembled either the deadly breath of Pluto/Hades, the Greek god of the kingdom of the death or even more plausible the breath of the frightening hellhound Kerberos guarding the entrance to hell. Furthermore, these places were also the sites were Pluto abducted Persephone, the daughter of Demeter (see also a similar site in Greece, Eleusis; Von Uxkull 1957). The already terrifying impression of Hadean entrances is sometimes corroborated by evaporating steam pouring out of holes and cracks on cold days. This is also true for the Plutonium at the Sanctuary of Hierapolis; a hot carbonate-rich, belowground creek is the cause of it (Fig. 2b).

It is also known from other authors that sacred Greek and Roman oracles and temples were often located on or close to geologically active zones (De Boer et al. 2001; Etiope et al. 2006; Piccardi et al. 2008; Mariolakos et al. 2010).

Understanding the principle of sacrificing bulls, rams, and birds

The toxic atmosphere around these sites lead Greeks and Romans to believe that these places, called mephitic caves or mephitic locations, were the entrance to the ancient underworld, the Hades, the ancient gate to hell (Plutonium, Charonium; see Piccardi 2007; Cassius Dio, n.d, Plinius, n.d, Strabo, n.d). As there were celebrations and festivities, ancient tourists, pilgrims, and spectators were attracted. Even at this time, tourism was sophisticated as not only accommodation and food was provided, but also living animals for sacrifice and other attractions were sold. The spectators of the religious presentations threw the freshly purchased birds down onto the antechambers floor (into the invisible gas lake) where the tiny creatures quickly died (Zwingmann 2012). Even more impressive were the effects of the Hadean atmosphere when larger animals were sacrificed. On special events and sacred days, bulls and rams were also ushered into the antechambers of the sanctuary. Young men had carried the sacrificial animal to the sanctuary (see coin found at Acharaka; Von Diest et al. 1913). While the bull was standing within the gas lake with its mouth and nostrils at a height between 60 and 90 cm, the large grown priests (Galli) always stood upright within the lake caring that their nose and mouth were way above the toxic level of the Hadean breath of death. It is reported that they sometimes used stones to be larger. The spectators could see that animals as strong, sturdy, and powerful as bulls died within minutes whereas the priests survived (for an overview see Pfanz et al. 2014). Many ancient writers discuss the survival of the Eunuch priests and state either the godlike powers of the Galli, the application of antidotes and/or other precautions to survive the deadly gas (Cassius Dio, Epitome 68.27.2 and 3; for details see Zwingmann 2012). It seems pretty clear that the priests were fully aware of the gas and also of its physicochemical properties. They were aware of the diurnal changes of the deadly vapors and they were aware of the varying height of the gas lake. For these reasons, it must be assumed that religious ceremonies including the sacrifice of animals were performed in the evening or early morning hours of calm days. When, on the other hand, the priest wanted to proof their fortitude or invincibility or just their similarity with the gods, they used midday hours for their performances. Then they could easily creep deep into the hole of the entrance to hell and by keeping their breath, they could even stay inside for a short while.


The measured 3He/4He isotopic ratios of dissolved gas phase taken from Hierapolis thermal water show a mixture of shallow (atmospheric) and deep components either of mantle and crustal origin (Fig. 8), with a significant contribution (about 75%) of mantle-derived helium considering a SCLM-type mantle end member (6.5 R/Ra). This is consistent with the active fault and fractures in the area.
Fig. 8

Helium and neon isotopic ratios given as R/Ra values and He/Ne relationships, respectively. The theoretical lines represent binary mixing trends of atmospheric helium with mantle-originated and crustal helium. The assumed end-member values for He isotopic ratios mark the mixing curves: crust 0.02 Ra and three different mantle-signatures: 8 Ra (MORB), 6.5 Ra (SECM), and 4 Ra for a contaminated/degassed mantle

Furthermore, our measurements strongly corroborate that at least in the case of Hierapolis, ancient writers like Strabo or Plinius described mystic phenomena very exactly without much exaggeration. More than 2000 years ago, these phenomena could not be explained scientifically but only by the imagination of supernatural forces from Hadean depths or well-meaning gods.

Measurements of diurnally changing concentrations and fluxes of the deadly gas emissions corroborate the ancient tales of asphyxiated bulls and rams and also the survival of the knowledgeable priests. In line with this, many examples for the description of the different oracle sites in proper Greece, Magna Graeca, and Asia Minor can be found. Nowadays, these phenomena can be analyzed by physicochemical means and explained with the knowledge in geology, chemistry, physics, and biology.



We want to thank the governorship of Denizli City for their kind help. We are indebted to Omer Faruk Gunay, vice governor of Eskisehir City, and Ismail Soykan, vice governor of Denizli City. The great help of the director of Hierapolis/Pamukkale is greatly acknowledged. The authors wish to extend their sincere thanks to the provincial culture and tourism directorate of Ankara and Denizli. Dr. Francesco Italiano kindly provided data about gas analysis from the grottos. Special thanks to Dr. Christiane Wittmann and Volker Wittmann for their excellent work in building a robust, continuously recording CO2 monitor. The authors are also grateful to Selami Yildirim and Mehmet Ergun from the Turkish State Meteorological Service. The authors are extremely thankful to Prof. Dr. Francesco D’Andria, who detected and excavated the new Plutonium at Hierapolis for his kind help providing a permission to work on the site, to his intelligent advices and to his permanent interest in the progress of our study.


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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Institute of Applied Botany and Volcano BiologyUniversity of Duisburg-EssenEssenGermany
  2. 2.Department of Geological EngineeringHacettepe UniversityAnkaraTurkey
  3. 3.Department of Geological EngineeringEskisehir Osmangazi UniversityMeselikTurkey
  4. 4.Department of Geological EngineeringPamukkale UniversityDenizliTurkey

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