Atoll (ring-shaped reef)

An atoll is a circular coral reef that surrounds a central lagoon. These structures form over millions of years as volcanic islands sink below the sea level. Most atolls exist in deep ocean waters where the surrounding seabed drops to depths exceeding 2,000 m. You often find them in the Maldives or the remote Tuamotu Archipelago. The reef rim provides protection from heavy swells. This creates calm lagoon waters for divers. However, the outer edge usually has strong currents and surge. I once dove a reef edge in the Chagos Archipelago where the drop-off went straight down to 400 m. Always check the local tide charts before planning a lagoon entry. High tides can push massive amounts of water through narrow reef passes.