What is the largest atoll in the world by total area?

The largest atoll in the world by total area is the Great Chagos Bank. It covers about 12,642 square kilometres in the Chagos Archipelago of the Indian Ocean.

The Great Chagos Bank is unusual because most of its reef structure is submerged. Only four reef areas emerge above the water, supporting a small number of low, sandy islands whose combined land area is roughly 5.6 square kilometres. This distinction explains why its total area is vastly larger than its visible land area.

The bank lies about 500 kilometres south of the Maldives. It is sometimes confused with the Great Barrier Reef, but the Great Barrier Reef is a vast reef system rather than a single atoll. The Great Chagos Bank was first surveyed by Commander Robert Moresby of the Royal Indian Navy in 1837, and its reefs and islands have been protected as a strict nature reserve since 1998.