Aldabra is the world's second-largest coral atoll. What is the first?

The Great Chagos Bank is the largest atoll in the world, taking up a massive area of 12,642 square kilometers (4,881 square miles). Most of this atoll is located underwater, with the exception of a few small islets which have been designated as strict nature reserves by the World Commission on Protected Areas (WCPA) since 1998. It was first surveyed by Robert Moresby in 1839, who also surveyed the aforementioned North Bank. Great Chagos Bank is currently administered by the British Indian Ocean Territory, an overseas territory of the United Kingdom.