Nestled among the South Shetland Islands is Deception Island, easily recognised on a map by its horseshoe shape. Its collapsed volcanic caldera is breached at Neptune's Bellows and makes for one of the world's safest natural harbours, despite the volcano's periodic eruptions. 

Ships enter the relatively calm waters of Port Forster (12km/7.5mi wide) through the caldera's breach that is surrounded by snow-covered hills that reach 580m (1900ft). The island has an interesting history - it was a base for several early exploratory missions - and is still a disputed territory between the Argentinians and the British, however nowadays they seem to get along.

Part of what brings the tourist ships here is that the volcanic activity thermally heats the waters of Pendulum Cove (so-called because of the British pendulum and magnetism experiments held there last century) and you can take a dip. It's not deep enough for swimming and most tourists don the togs more for the photograph and the story afterwards than for breaststroking. You do have to be careful, however, because if you move even a metre from the warm water you might find your skin blistering from a near-boiling patch or goosebumping from an unheated patch. 

There are large colonies of chinstrap penguins on the exterior coast, but few marine animals enter the harbour because there are numerous volcanic vents that heat the water to several degrees above the sea surrounding the island.