Deception Island, one of the main islands of the South Shetland chain, is actually the sunken caldera of a semi-active volcano. Approached through a narrow, hidden channel called Neptune's Bellows, the entrance to this perfect harbour is spectacular. The mile-wide flooded caldera bay is ringed by bubbling fumaroles, a reminder of the volcanic activity which took place in 1967-1970, wiping out two scientific stations. It is also the site of long-closed Hektor Whaling Station, and home to a huge Chinstrap penguin rookery (the largest in Antarctica). Go ashore to enjoy the dramatic beauty of this thermal hot spot and swim at Pendulum Cove - where the sea is heated to comfortable swimming temperatures.