The Porcupine Bank is a block of continental crust that became separated from the rest of the European continent by a failed rift during the opening phase of the North Atlantic, which began about 250 million years ago. The Porcupine Bank separates the Porcupine Seabight from the Rockall Trough. In the northeast, the Porcupine Bank is still connected with the Irish continental margin by the underlying structure of the Slyne Ridge. The summit of the Porcupine Bank is shallow lying at 145 m water depth and is generally broad and flat, although some structures from the underlying basement rocks can be seen emerging in places. The eastern slope, towards the Porcupine Seabight, is gentle whereas the southern, western and northern slopes towards the Rockall Trough are steep. Along the western and northern Porcupine Bank, the slope-break from the flat summit area onto the steep slopes occurs at a remarkably consistent water depth of approximately 450 m bsl and is generally marked by a prolonged escarpment. The western and northern slopes of the Porcupine Bank facing the Rockall Trough are characterised by irregularly spaced canyons and the south-western slope of the Porcupine Bank is especially steep and eroded (Fig. 14.1).