The Great Chagos Bank, in the Chagos Archipelago, about 500 km (310 mi) south of the Maldives, is the largest atoll structure in the world, with a total area of 12,642 km2 (4,881 sq mi). The atoll is administered by the United Kingdom through the British Indian Ocean Territory (BIOT).
Despite its enormous size, the Great Chagos Bank is largely a submarine structure.
There are only four emerging reefs, mostly located on the western rim of the atoll, except for lonely Nelson Island which lies wholly isolated in the middle of the northern fringe. These reefs have seven or eight individual low and sandy islands, with a total land area of about 5.6 km2 (2.2 sq mi).
All islands and their surrounding waters are a Strict Nature Reserve since 1998. The total length of the eastern and southern expanses of the bank, as well as the reefs in its central area are wholly submerged.
According to en.wikipedia