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, with a distance of 1,120 km (700 mi) southwest of the capital, Victoria, on Mahé Island.

Extremely isolated, Aldabra is almost untouched by humans. Aldabra atoll is closer to the coast of Africa 630 km (390 mi) than to Mahé, and is in the most southwesterly part of the Seychelles. It is 407 km (253 mi) northwest of Madagascar and 440 km (270 mi) from Moroni on the Comoro Islands. 

The atoll is the largest raised coral reef in the world with an elevation of 8 metres (26 ft); and the second-largest atoll in the world after Kiritimati Atoll. Aldabra atoll is 34 kilometres (21 mi) long (in east–west direction) and 13 kilometres (8.1 mi) wide.

It has a large shallow lagoon, 196 square kilometres (76 sq mi) in area, of which roughly two thirds is dry during low tide. The lagoon is encircled by fringing coral reef. Around the rim of the lagoon are the larger islands of the Atoll. The total land area of the Atoll is 155.4 square kilometres (60.0 sq mi). The size including the lagoon is 380 square kilometres (150 sq mi). 

The outside rim of the atoll has three passages which connect to the lagoon, which is 6–10 kilometres (3.7–6.2 mi) in width as it opens to the sea. The depth of water in the lagoon averages about 5 metres (16 ft); however, the passages that open to the sea are up to 20 metres (66 ft) deep and strongly affected by tidal currents.

According to en.wikipedia