Asia's smallest and least-known nation, the Republic of Maldives, lies scattered from north to south across a 750-kilometre sweep of the Indian Ocean 500 kilometres south-west of Sri Lanka. More than 1000 islands, together with innumerable banks and reefs, are grouped in a chain of nineteen atolls which extends from a point due west of Colombo to just south of the equator. The atolls, formed of great rings of coral based on the submarine Laccadive-Chagos ridge, vary greatly in size. Some are only a few kilometres square, but in the far south the great atoll of Suvadiva is sixty-five kilometres across, and has a central lagoon of more than 2000 square kilometres. The northern and central atolls are separated from each other by comparatively narrow channels of deep water, but in the south Suvadiva is cut off by the eighty-kilometre-wide One-and-a-half-Degree Channel. Addu Atoll is still more isolated, being separated from the atoll of Suvadiva by the seventy-kilometre-wide Equatorial Channel.