The Chagos Archipelago ( /ˈtʃɑːɡoʊs/ or /ˈtʃɑːɡəs/; formerly Bassas de Chagas and later also Oil Islands, known as Foalhavahi (ފޯޅަވަހި) in Dhivehi, as Phehandweep (फेहंद्वीप) in Hindi and other North Indian languages, and as Paeikaana Theevukal (பேகான தீவுகள்) in Tamil), is a group of seven atolls comprising more than 60 individual tropical islands in the Indian Ocean; situated some 500 kilometres (310 mi) due south of the Maldives archipelago. This chain of islands are the southernmost archipelago of the Chagos-Laccadive Ridge, a long submarine mountain range in the Indian Ocean. The Chagos also form a terrestrial ecoregion together with the Maldives and the Lakshadweep. The islands and their surrounding waters are also a vast oceanic Environment Preservation and Protection Zone (EPPZ) (Fisheries Conservation and Management Zone (FCMZ) of 544,000 square kilometres (210,000 sq mi)), an area twice the size of the UK's land surface.
Officially part of the British Indian Ocean Territory, the Chagos were home to the Chagossians for more than a century and a half until the United Kingdom evicted them in the early 1970s in order to allow the United States to build a military base on Diego Garcia, the largest of the Chagos Islands. Since 1971, only the atoll of Diego Garcia is inhabited, and only by military and civilian contracted personnel.
The sovereignty of the Chagos Archipelago is being disputed between the UK and Mauritius. The archipelago was split from the Territories of Mauritius prior to Mauritian independence in 1968.
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 in the Maldives there is not a clearly discernible pattern of arrayed atolls, 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). It also has one of the healthiest reef systems in the cleanest waters in the world, supporting 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.
On 1 April 2010, the British government Cabinet established the Chagos Archipelago as the world's largest marine reserve. At 640,000 km2, it is larger than the country of France or the state of California. The protection of the marine reserve will be guaranteed for the next five years thanks to the financial support of the Bertarelli Foundation. The setting up of the Marine Reserve would appear to be an attempt to prevent any resettlement by the evicted natives in the 1960s and 70s. Leaked US Cables have shown the FCO suggesting to the US counterparts that setting up a protected no-take zone would make it "difficult, if not impossible" for the islanders to return. The reserve was then created in 2010.