Great Chagos Bank

The Great Chagos Bank, in the Chagos Archipelago, about 500 km South of the Maldives, is the largest atoll structure in the world, with a total area of 12 642 km²[1]. There are seven or eight individual islands on the rim of the atoll, one in the North (Nelson's Island) and the others on the Eastern rim. The total land area of the islands is about 4.5 km². The Atoll is administered by the UK. The individual islands, starting in the South clockwise, are:

• Danger Island (slightly more than 2 km long from North to South, up 1 km wide, land area 0.66 km², vegetated with palm trees up to 12 m high, Strict Nature Reserve since 1998)
• Eagle Islands
  • Ile Aigle (Eagle Island, vegetated with high coconut trees, land area 2.45 km²
  • Cow Island (Ile aux Vaches, vegetated with trees, land area 0.18 km², Strict Nature Reserve since 1998)
• Three Brothers (Trois Fréres) and Resurgent Islands (vegetated with high coconut trees, land area 0.4 km², Strict Nature Reserve since 1998)
  • Ile du Sud (South Island, largest of the group), 0.23 km²
  • Ile du Mileu (Middle Island), 0.08 km²
  • unnamed rocky islet
  • Ile du Nord (North Island), 0.06 km²
• Nelsons Island (2 km long from East to West, up to 1 km wide, land area 0.81 km², 3 m high, bushy vegetation, Strict Nature Reserve since 1998)

Cartography of the Submerged Reefs

The Great Chagos bank was surveyed for the first time by Commander Robert Moresby of the Indian Navy in 1837. Although the charts of "normal" atolls like Peros Banhos and Diego Garcia were relatively accurate, the cartography of the vast sunken reefs forming the Great Chagos Bank proved quite a challenge. The real shape of these sunken reefs was known only when satellite imagery became available in the latter part of the 20th century. Moresby's original hydrographic drawings were somewhat at variance with the true shape of the submerged reef, especially in areas where there were no emerging islands close by, like in the South east of the bank. The outlines of the first hydrographic surveys are still marked in present-day navigational maps of the Chagos with a dotted line and the legend "existence doubtful".