Atoll

An atoll is a ring-shaped or horseshoe-shaped coral reef enclosing a shallow lagoon. According to Darwin's subsidence theory, coral reefs develop around volcanic islands, and as the volcano sinks, the reef remains as an atoll.

Countries made up largely of atolls include Maldives, Marshall Islands (29 atolls), Kiribati, and Tuvalu. Because atoll land lies only slightly above sea level, these nations are known to be vulnerable to sea-level rise.

A hint that a country consists mostly of atolls points to Maldives, Marshall Islands, Kiribati, and Tuvalu as the main candidates. Combined with the Indian Ocean, Maldives becomes the strongest candidate.