Great Chagos Bank

The Great Chagos Bank is the largest submerged atoll on Earth, covering an area of approximately 12,642 square kilometres, and its rim is almost entirely submerged at high tide, creating an enormous shallow lagoon that is largely inaccessible to conventional vessels and visible from satellite imagery as a vast pale disc in the deep blue of the Indian Ocean.

Several small sand cays and rock outcrops protrude above the waterline at the northern and western edges of the rim, providing the only habitable ground on the bank and hosting small colonies of seabirds and a scattering of salt-tolerant vegetation that manages to survive on the most marginal of land surfaces.

The lagoon inside the rim is extraordinarily shallow in places, often less than two metres deep, and the coral formations that grow up to just below the surface create a hazard for navigation that has historically kept the area free from the anchor damage and groundings that have scarred reefs elsewhere in the Indian Ocean.

The surrounding ocean waters are cold upwelling zones driven by the banks, bringing nutrients to the surface and attracting large concentrations of pelagic fish including yellowfin tuna, bigeye tuna and billfish that sustain significant artisanal and occasionally illegal fishing activity in the waters beyond the reef rim.

The bank is administered as part of the British Indian Ocean Territory and was included in the 2010 Marine Protected Area designation, though enforcement over such an enormous and remote area is difficult and the bank has historically been subject to irregular incursions by fishing vessels.

Scientific surveys conducted from the Chagos Archipelago research expeditions have recorded exceptional fish biomass on the outer reef slopes, with groupers, snappers and jacks present in densities comparable to the most productive reefs in the Pacific, and the inner lagoon reefs, while more sediment-stressed, still support diverse communities of hard and soft corals.

The Great Chagos Bank represents one of the last major unspoiled marine wilderness areas in the Indian Ocean, and its scale — larger than many countries — means that it functions as an ecological anchor for the entire archipelago, seeding larval populations that drift south to the other atolls and reefs.

The bank is so large that it creates its own weather system: cloud formations build over the shallow lagoon during the day due to solar heating of the extensive shallow water, and these clouds can generate localised squalls and rain showers even when the surrounding ocean is clear, a phenomenon that is visible from passing vessels and aircraft as a persistent white mass above the bank.

The Great Chagos Bank is the largest submerged atoll in the world and the scale of its lagoon, reef rim and surrounding ocean creates a marine environment that is essentially without parallel in the Indian Ocean in terms of size and intactness.

The bank and its surrounding waters support fish biomass and coral cover that rivals the most celebrated Pacific reef systems, making it a reference site for marine ecologists studying baseline reef health.

The near-total absence of permanent human presence means that the bank offers a wilderness experience of a type that is vanishingly rare in the modern ocean.