Deception Island

Everything starts with the passage in. Neptune's Bellows, on the southeast side of the island, is 560 m wide and is the only entrance to Port Foster, and the wind that gave the channel its name gusts in and out of it hard enough to be felt from the deck. Inside, the caldera opens into a basin roughly 10 by 7 km across with a flat floor lying as deep as 170 m, far below anything a diver goes near; polar dive operations here work to a maximum of around 20 m. Zodiacs, which is how every dive on this coast is launched, work the shallow margins instead.

The shoreline is black volcanic sand, and in places it is warm enough that digging into the beach produces a hole that fills with water you can bathe in. The harbour floor is why two parts of Port Foster are collectively protected as Antarctic Specially Protected Area No. 145: the benthic zone here is naturally disturbed by the volcanic activity, which makes it of great ecological interest and puts limits on where anyone may get in the water. Confirm with the expedition leader which parts of the basin fall inside the protected areas before a dive is planned.

Wildlife is largely a topside matter. Baily Head, on the island, holds a breeding colony of about 100,000 pairs of chinstrap penguins, and it is visited as a landing rather than as a dive. Inside the caldera the interest is the volcanic ground itself, the sand, the heat still in it, and a seabed community that is periodically wiped over by the volcano rather than left to settle.