Or, as it’s actually called, an atoll: Boats sheltering in an atoll. A photo posted by Kjell Lindgren (@astro_kjell) on Nov 29, 2015 at 2:34pm PST The birth of ring-shaped islands can take thousands of years. NOAA’s National Ocean Service explains (with a handy animation): Corals … begin to settle and grow around an oceanic island forming a fringing reef.