Changu Island, also known as Prison Island, is about a 30-minute boat ride from Stone Town. The water around the island is crystal-clear and cerulean blue, which makes snorkeling here hard to resist, but a visit also lets you gain an understanding of a grim period of Zanzibar history. In 1860, the small island was used as a prison for disobedient slaves. By 1893, construction of an actual prison complex was completed, but instead of housing prisoners, it was used to quarantine yellow fever cases. Nowadays, the island is home to some endangered Aldabra giant tortoises, given to the island in 1919 by the British governor of the Seychelles. Some of those original tortoises are supposedly still alive here to this day!