Atoll

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

Countries composed of atolls include Maldives (about 1,200 islands, 26 atolls), Marshall Islands (29 atolls), Kiribati (32 atolls), and Tuvalu (6 of 9 islands are atolls). These nations have maximum elevations of just a few meters, making them most vulnerable to sea-level rise.

In GeoHint, composed of atolls or maximum elevation below 5m narrows to Maldives, Marshall Islands, Kiribati, or Tuvalu. Atoll + Indian Ocean means Maldives alone.