The Chagos Archipelago (, ) or Chagos Islands (formerly , and later the Oil Islands) is a group of seven atolls comprising more than 60 islands in the Indian Ocean about south of the Maldives archipelago. This chain of islands is the southernmost archipelago of the Chagos–Laccadive Ridge, a long submarine mountain range. To the north are the Salomon Islands, Nelsons Island and Peros Banhos; to the southwest are the Three Brothers, Eagle Islands, Egmont Islands and Danger Island; southeast of these is Diego Garcia, by far the largest island. All are low-lying atolls, save for a few extremely small instances, set around lagoons.

The islands were first permanently settled by Europeans. From 1715 to 1810, the Chagos Islands were part of France's Indian Ocean possessions, administered through Isle de Francewhich was a colony of France (later renamed Mauritius). Under the Treaty of Paris in 1814, France ceded and the Chagos Islands to the United Kingdom.

In 1965, the United Kingdom split its administration of the Chagos Archipelago away from Mauritius and into the British Indian Ocean Territory (BIOT), to allow the United States to build Naval Support Facility Diego Garcia, which operates under a special agreement allowing significant US military presence. The islands were formally established as an overseas territory of the United Kingdom on 8 November 1965.

The only permanent inhabitants are employees of the military, including civilian contracted personnel, on Diego Garcia. The Chagos Islands are the homeland of the Chagossians, a Bourbonnais Creole–speaking people, though since 1971 no Chagossians have been allowed to live there. The United Kingdom expelled the entire Chagossian population from the archipelago at the request of the United States between 1967 and 1973. The main forcible removal of Diego Garcia's population took place in July and September 1971. In 2026, a group of Chagossians returned to the archipelago without government permission in an effort to reestablish the settlement on Île du Coin, and began legal action to establish their right to stay permanently.

Mauritius engaged in a sovereignty dispute with the UK, claiming the Chagos Archipelago as part of Mauritius. The International Court of Justice (ICJ) in 2019 and the International Tribunal for the Law of the Sea (in 2021) both stated that the UK had an obligation to return the islands to Mauritius. In October 2024, the British government announced it would transfer the Chagos Islands to Mauritius subject to finalisation of a treaty. The transfer agreement was signed on 22 May 2025, with the provision that the island of Diego Garcia would be leased back to the UK for at least 99 years. The UK government expected the treaty to be ratified sometime in 2025, but the legislation is indefinitely on hold due to both US opposition, as well as domestic political opposition in the UK. Opinion polling in the UK has consistently shown more opposition to the deal than support from the UK population. The UN Committee on the Elimination of Racial Discrimination has expressed "deep concern" at the terms of the deal. Opponents of the deal have cited Article 298 (b) of the United Nations Convention on the Law of the Sea (UNCLOS) as giving the British government an invokable legal exemption from "disputes concerning military activities, including military activities by government vessels and aircraft engaged in non-commercial service", and have argued that the British government's case was misleading.

Geography

The archipelago is about south of the Maldives, east of the Seychelles, north-east of Rodrigues Island (Mauritius), west of the Cocos (Keeling) Islands, and north of Amsterdam Island (France).

The land area of the islands is , the largest island, Diego Garcia, having an area of . The total area, including lagoons within atolls, is more than , of which are accounted by the Great Chagos Bank, the largest acknowledged atoll structure of the world (the completely submerged Saya de Malha Bank is larger, but its status as an atoll is uncertain). The shelf area is , and the Exclusive Economic Zone, which borders the corresponding zone of the Maldives in the north, has an area of (including territorial waters).

The Chagos group is a combination of different coralline rock structures topping a submarine ridge running southwards across the centre of the Indian Ocean, formed by volcanoes above the Réunion hotspot. Unlike the Maldives, there is no clearly discernible pattern in the atoll arrangement, which makes the whole archipelago look somewhat chaotic. Most of the coralline structures of the Chagos are submerged reefs.

The Chagos contain the world's largest coral atoll, The Great Chagos Bank, which supports half the total area of good quality reefs in the Indian Ocean. As a result, the ecosystems of the Chagos have so far proven resilient to climate change and environmental disruptions.

The largest individual islands are Diego Garcia (), Eagle (Great Chagos Bank, ), (Peros Banhos, ), Eastern Egmont (Egmont Islands, ), Île du Coin (Peros Banhos, ) and (Salomon Islands, ).

In addition to the seven atolls with dry land reaching at least the high-water mark, there are nine reefs and banks, most of which can be considered permanently submerged atoll structures. The number of atolls in the Chagos Archipelago is given as four or five in most sources, plus two island groups and two single islands, mainly because it is not recognised that the Great Chagos Bank is a huge atoll structure (including those two island groups and two single islands), and because Blenheim Reef, which has islets or cays above or just reaching the high-water mark, is not included. Features are listed in the table from north to south:

Atoll/Reef/Bank (alternate name) type Area (km2) number of islands Location Land Total 0 unnamed bank submerged bank – 3 – 1 Colvocoresses Reef submerged atoll – 10 – 2 Speakers Bank unvegetated atoll 0.001 582 1) 3 Blenheim Reef (Baixo Predassa) unvegetated atoll 0.02 37 4 4 Benares Shoals submerged reef – 2 5 Peros Banhos atoll 9.6 503 32 6 Salomon Islands atoll 3.56 36 11 7 Victory Bank submerged atoll – 21 – 8a Nelson Island parts of mega-atoll Great Chagos Bank 0.61 12642 1 8b Three Brothers (Trois Frères) 0.53 3 8c Eagle Islands 3.43 2 8d Danger Island 1.06 1 9 Egmont Islands atoll 4.52 29 7 10 Cauvin Bank submerged atoll – 12 – 11 Owen Bank submerged bank – 4 – 12 Pitt Bank submerged atoll – 1317 – 13 Diego Garcia atoll 32.8 174 42) 14 Ganges Bank submerged atoll – 30 – 15 Wight Bank 3 16 Centurion Bank 25 Chagos Archipelago Archipelago 56.13 15427 64 04°54' to 07°39'S 70°14' to 72°37' E 1) a number of drying sand cays 2) main island and three islets at the northern end

Resources

The main natural resources of the area are coconuts and fish. The licensing of commercial fishing used to provide an annual income of about for the British Indian Ocean Territory authorities. No licenses have been given since October 2010, however, and the last expired after the creation of the no-take marine reserve.

All economic activity is concentrated on the largest island of Diego Garcia, where joint UK–US military facilities are located. Construction projects and various services needed to support the military installations are done by military and contract employees from the UK, Mauritius, the Philippines, and the US. There are currently no industrial or agricultural activities on the islands. All the water, food and other essentials of daily life are shipped to the island. An independent feasibility study led to the conclusion that resettlement would be "costly and precarious". Another feasibility study, commissioned by organisations supporting resettlement, found that resettlement would be possible at a cost to the British taxpayer of . If the Chagossians return, they plan to re-establish copra production and fishing, and to begin the commercial development of the islands for tourism.

Until October 2010, Skipjack (Euthynnus pelamis) and yellowfin tuna (Thunnus albacares) were fished for about two months of the year as their year-long migratory route takes them through Chagos waters. While the remoteness of the Chagos offers some protection from extractive activities, legal and illegal fishing have had an impact, as well as considerable poaching of turtles and other marine life. Sharks, which play a vital role in balancing the food web of tropical reefs, have suffered sharp declines from illegal fishing for their fins and as bycatch in legal fisheries. Sea cucumbers, which cleanse sand, are poached to feed Asian markets.

Climate

The Chagos Archipelago has a tropical oceanic climate; hot and humid but moderated by trade winds. Climate is characterised by plenty of sunshine, warm temperatures, showers and light breezes. December through February is considered the rainy season (summer monsoon); typical weather conditions include light west-northwesterly winds and warmer temperatures with more rainfall. June to September is considered the drier season (winter), characterised by moderate south-easterly winds, slightly cooler temperatures and less rainfall. The annual mean rainfall is , varying from during August to during January.

History

Early history According to Southern Maldivian oral tradition, local traders and fishermen were occasionally lost at sea and became stranded in one of the islands of the Chagos. Eventually they were rescued and brought back home.Romero-Frias, Xavier (2012). Folk tales of the Maldives, NIAS Press, , . However, these islands were judged to be too far away from the Maldives to be settled permanently by Maldivians. Thus, for many centuries, the Chagos were ignored by their northern neighbours.

16th to 19th century

The first Europeans to become aware of the archipelago were Portuguese explorers. Although the Portuguese navigator Pedro de Mascarenhas (1470 – 23 June 1555) is credited with having encountered the islands during his voyage of 1512–13, there is little corroborative evidence; cartographic analysis points to 1532 or later. Portuguese seafarers named the group ,Huddart, Joseph, et al. The oriental navigator, or, New directions for sailing to and from the East Indies, . () referring to the Holy Wounds of the crucifixion of Jesus. They also named some of the atolls, such as Diego Garcia and Peros Banhos Atoll, mentioned as Pedro dos Banhos in 1513 by Afonso de Albuquerque.Purdy, John, Memoir, descriptive explanatory, accompany new chart Ethiopic or southern Atlantic Ocean, western coasts South-America, Cape Horn Panama, . This lonely and isolated group, economically and politically uninteresting to the Portuguese, was never made part of the Portuguese Empire.Boxer, C R., The Portuguese Seaborne Empire, 1415–1825, Knopf, 1969, .

The earliest description of the Chagos was written by Manoel Rangel, a castaway from the Portuguese ship Conceição which ran aground on the Peros Banhos reefs in 1556.Bernardo Gomes de Brito. História trágico-marítima. Em que se escrevem chronologicamente os Naufragios que tiverao as Naos de Portugal, depois que se poz em exercicio a Navegaçao da India. Lisboa 1735

The oldest known written document claiming the Chagos is attributed to King Hassan IX of the Maldives in the year 1561. The French were the first European colonial power to lay claim to the Chagos after they settled (now Réunion, in 1665) and Isle de France (now Mauritius, in 1715). The French began issuing permits for companies to establish coconut oil plantations on the Chagos in the 1770s.

On 27 April 1786 the Chagos Islands and Diego Garcia were claimed for Great Britain. However, the territory was ceded to Britain by treaty only after Napoleon's defeat, in 1815. The Chagos were governed from Mauritius, which was by that time also a British colony.