Chagos Archipelago

Chagos Archipelago

The Chagos Archipelago, formerly Bassas de Chagas and later also Oil Islands, 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 is 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 and allowed 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 United Kingdom excised the archipelago from Mauritian territory prior to Mauritius’ independence.

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. It doubled the total area of environmental no take zones worldwide.
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.



The entire land area of the islands is a mere 56.13 km², with the largest island, Diego Garcia, having an area of 32.5 km². The total area, including lagoons within atolls, however, is more than 15,000 km², of which 12,642 km² 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 20,607 km², and the Exclusive Economic Zone, which borders the corresponding zone of the Maldive Islands in the north, has an area of 639,611 km² (including territorial waters).

The largest individual islands are Diego Garcia (32.5 km²), Eagle (Great Chagos Bank, 3.1 km²), Île Pierre (Peros Banhos, 1.40 km²), Eastern Egmont (Egmont Islands, 2.17 km²), Île du Coin (Peros Banhos, 1.32 km²) and Île Boddam (Salomon Islands, 1.27 km²).

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 recognized that the Great Chagos Bank is a huge atoll structure (including those two island groups and two single islands), and because it is not recognised that Blenheim Reef has islets or cays above or just reaching the high-water mark.

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.


Blenheim Reef

Blenheim Reef is a partly submerged atoll structure in the Chagos Archipelago, Indian Ocean. It includes the coral reef of Baxio Predassa in its southeastern rim, plus another completely submerged part. It is located in the northeastern part of the Chagos Archipelago. It measures almost eleven kilometres (north–south) by more than four kilometres (east–west), with a total area of 36.8 square kilometres, including the lagoon of 8.5 km², the difference being accounted for the mostly by the reef flat.

Only on the eastern side, there are a few sand cays above the water. The largest of them is East Island, which is not quite 200 metres long and 70 metres wide. The other islands in the group are North, Middle and South.
Only a few grasses grow on the island. The lagoon is up to 18 metres deep and encumbered with rock. The fringing coral reef has a wide passage in the southwest. The closest land is Takamaka Island in the Salomon Islands Atoll, about 20 kilometres to the southwest.


The atoll is thought to have been discovered around 1570 by Portuguese sailors, which is why it is also known under its Portuguese name Baxio Predassa (a corruption of Baixo Predassa). The present name comes from the ship Blenheim of the East India Trading Company, which was lost in 1799 to the reef.

From 1845 to 1860, guano and phosphate were mined. This venture was then given up due to inadequate transport facilities. Around 1880, the Indian Ocean Fruit Company attempted to plant palm trees on the island, but all seedlings were washed into the sea during a storm. On 29 June 1975, the atoll was incorporated into the Chagos Archipelago and claimed by Mauritius. Before that day, it was not claimed by any nation.

In total, there have been 57 ship wrecks around Blenheim Reef, with an estimated 200 lost lives.


Baxio Predassa is the name given to the islands that emerge from the eastern rim of the reef. When first surveyed by Robert Moresby, there were many more islands in Baxio Predassa.
These have, however, been eroded or submerged and only four islets remain.

The group consists of:

  • Ile du Nord, the northernmost island
  • Ile du Milieu, surrounded by a reef
  • Ile de l’Est, the largest island and the easternmost island in the atoll. The island has only grasses growing on it, though there was once an unsuccessful attempt to plant palms on the island.
  • Ile du Sud, the smallest and southernmost islet, it is unvegetated.


Danger Island

Danger Island is the westernmost and the southernmost island of the Great Chagos Bank, which is the world’s largest coral atoll structure, located in the Chagos Archipelago.
This is a long and flat island, covered with tall coconut trees. Its name probably derives from the lack of a safe anchorage, which rendered every visit to this island dangerous for the ship and crew.

There was never a permanent settlement on Danger Island, even at the time that the Chagos were inhabited (between mid 18th and mid 20th century). However, occasionally plantation workers from other islands would be brought to this island to collect coconuts.


Egmont Islands

Egmont Islands – Click to enlarge

The Egmont Islands or Six Iles are one of the few emerged coral atolls that make up the Chagos Archipelago. This small atoll lies less than 10 km South of the submerged coral reef of the Southern rim of the Great Chagos Bank.
The nearest Island is Danger Island on the Great Chagos Bank, less than 30 km due North. Its total size is 29 km², including the lagoon and the fringing coral reef. The land area totals about 4 km². The largest island is “Île Sud-Est” (Eastern Egmont), where the settlement was located, with an area of 1.5 km². While “Île Lubine” is similar in size, the other islets are smaller. All islands are covered with coconut trees.
Ile Sudest, the largest island, was first settled in the last half of the 1700s, at the time when the other Atolls of the Chagos were settled by the French.

Commander Robert Moresby made a survey of this atoll and charted it in 1838. At that time there was a coconut plantation on this atoll.
A causeway had been built linking the islands and some land reclamation was done. Eventually some of the islands merged with each other.
The Egmont Islands were no longer inhabited in the second half of the twentieth century, at the time that the Chagossians or Ilois were evicted from the Chagos.
There are two passages into the lagoon along the Northern Rim, Fausse Passe in the Northeast and a wider passage in the Northwest. The Egmont islands are one of the favorite anchoring spots for itinerant yachtsmen passing through the Chagos.

West of Ile aux Rats there was a small islet known as Ilot aux Rats which has now merged with Ile aux Rats.
Ile Cipaye is also spelled ‘Sipaille’ and ‘Cipaille’. Isle Carre Pate is also spelled ‘Carpathe’. On some Maritime Charts Ile Takamaka is spelled as ‘Tattamucca’.
West of Ile aux Rats there was a small islet known as Ilot aux Rats which has now merged with Ile aux Rats. Ile Cipaye is also spelled ‘Sipaille’ and ‘Cipaille’. Isle Carre Pate is also spelled ‘Carpathe’. On some Maritime Charts Ile Takamaka is spelled as ‘Tattamucca’.

The individual islands are all on the Southern rim of the coral reef.
They are, from Southeast to Northwest:

  • Île Sud-Est
  • Île Takamaka
  • Île Carre Pate
  • Île Lubine
  • Île Cipaye
  • Île aux Rats


Salomon Islands

Soloman Islands – Click to enlarge

The Salomon Islands is a small atoll of the British Indian Ocean Territory. It is located in the Northeast of the Chagos Archipelago, between Blenheim Reef and Peros Banhos. The main islands in the group are Ile Boddam, with the former main settlement, and a land area of 1.08 km², and Ile Anglaise (0.82 km²), both on the western rim of the reef. There were smaller settlements of Chagossians in Fouquet (0.45 km²) and Takamaka (0.48 km) Islands. Île de la Passe is 0.28 km² in area, and Île Mapou 0.04 km². The remaining islets are much smaller. The total land area is about 3.5 km².
There is a passage into the lagoon, named Baie de Salomon, on the Northern side, between Ile Anglaise and Ile de la Passe. The Salomon Islands are one of the favorite anchoring spots for itinerant yachtsmen passing through the Chagos.
Now uninhabited, the islands are overrun by low jungle between the coconut trees and it is hard to find the traces of the former settlements.


This atoll was settled in the last half of the 18th century by coconut plantation workers from Mauritius (then known as Ile de France). Little is known about the condition of the workers who were mostly of African origin and most probably living in conditions close to slavery. The company exploiting the plantation was called the Chagos Agalega Company.

The Salomon Islands were surveyed in 1837 by Commander Robert Moresby of the Indian Navy on the HMS Benares. Moresby’s survey produced the first detailed map of this atoll.
They were surveyed again in 1905 by Commander B.T. Sommerville on the HMS Sealark, who drew a more accurate map. Some of the Salomon Islands were inhabited by the Chagossians, but at the time that the British Government decided to empty the Chagos of local inhabitants only Ile Boddam was inhabited.

The inhabitants of the Salomon Islands, numbering about 400, were forcefully evicted by the British and resettled in Mauritius. Ile Boddam had a jetty, shops, offices, a school, a church and a villa where the plantation manager lived. All these buildings are hidden by thick jungle now. The tanks still collect water which is used by yachtsmen to replenish their supplies.


The individual islets of the atoll are, starting in the North, clockwise:

  • Île de la Passe
  • Île Mapou
  • Île Takamaka
  • Île Fouquet
  • Île Sepulture
  • Île Jacobin
  • Île du Sel
  • Île Poule
  • Île Boddam
  • Île Diable
  • Île Anglaise