The ice sheet of McMurdo sound grinds into Tent Island as it slowly creeps seaward. The resulting pressure produces many cracks and ice crevices that sculpt the overhead ice layer into a 'cavernous' appearance. One of the interesting things about the marine invertebrate community in McMurdo Sound is the high prevalence of large sponge species. The Delbridge Islands are essentially the remains of an old caldera rim from an ancient volcano. They are due west of Mt. Erebeus, which is an active volcano here that has probably developed from the same geothermal activity that produced the Delbridge caldera rim.
Bottom contour has a steep slope as the sides drop quickly off into the deep caldera.
The primary ice hole through which the divers enetered the water is in between these two, but darkened because there is a dive-hut over it that obstructs the direct sunlight.