Côn Đảo is an island off the southern coast of Vietnam. Con Dao is also an island of immense natural beauty with forested hills, deserted sandy beaches and extensive coral reefs making for some excellent diving.
Con Dao served as a prison island for political prisoners during the French colonial era, when it was known as Poulo Condore, and in later years the Saigon regime imprisoned opponents of the regime in the infamous cells known as the "tiger cages". The old prison buildings are still standing and are open to the public as is a small museum tracing the island's history.
Con Dao has the best diving in Vietnam. You can dive most times of the year as there are 16 islands to choose from and you can get good visibility pretty much most of the year. Having dive insurance is mandatory in Con Dao due to its remote location.