Aldabra Islands, atoll, one of the world’s largest, in the Indian Ocean about 600 miles (1,000 km) southwest of the Seychelles group, and part of the Republic of Seychelles. The Aldabras, together with Farquhar and Desroches islands and the Chagos Archipelago, formed part of the British Indian Ocean Territory from 1965 to 1976. The Aldabra Islands were formerly under the hegemony of Seychelles, to which they were returned in June 1976, when that country gained its independence. They constitute an oval atoll (19 miles [30.5 km] long and 8 miles [13 km] wide) enclosing a large but shallow lagoon. Channels divide the ring into four low islands that rise to about 100 feet (30 meters) above the sea and are called South Island (Grand Terre; the largest), West Island (Picard), Polymnie, and Middle Island (Malabar). In common with about half the other islands of Seychelles, the Aldabra Islands are volcanic in origin. Mangrove forests are found on the islands.

The Aldabras were once rich in guano (deposits formed from bird droppings), but it was almost all collected and sold for fertilizer in the early 1950s. South Island is famous for its giant tortoises, and a nature reserve established in 1976 completely protects them and other animals. The Aldabras were designated a UNESCO World Heritage site in 1982. The islands were uninhabited in the late 20th century.