Aldabra is the world's second-largest coral atoll. It is situated in the Aldabra Group of islands in the Indian Ocean that are part of the Outer Islands of the Seychelles.

Due to its remoteness and inaccessibility, the atoll has remained largely untouched by humans for the majority of its existence. Aldabra contains one of the most important natural habitats for studying evolutionary and ecological processes. It is home to the largest giant tortoise population in the world.

Permanent staff conduct research to study Aldabra's biodiversity. Visits to the island by people other than the scientists and staff are strictly controlled and only guided tours are provided with prior permission. Cruises operated by several companies, along with dive boats, may visit the atoll on expedition tours.

Aldabra is classic coral atoll which has been built up from a seabed. It comprises four main islands of coral limestone separated by narrow passes and encloses a large shallow lagoon. Most of the land surface comprises ancient coral reef (about 125,000 years old) now raised above sea level, the rest being even older reef limestones. The lagoon contains many smaller islands and the entire atoll is surrounded by an outer reef.

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