Deception Island is a volcanic caldera formed by the collapse of a volcano on the island. The water in the caldera is several hundred feet deep but there is only a very narrow channel into the caldera. Along the shore there are areas where the water is warm enough to swim in, with the surrounding water at a brisk 34º F.

Addendum Feb. 3, 2007: This is where the M/S Nordkapp ran aground on submerged rocks at the entrance into the caldera, on its very next trip after ours.