Orbital View: An Ocean Halo

Or, as it’s actually called, an atoll:

Boats sheltering in an atoll.

A photo posted by Kjell Lindgren (@astro_kjell) on

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. It can take as long as 10,000 years for a fringing reef to form. Over the next 100,000 years, if conditions are favorable, the reef will continue to expand. As the reef expands, the interior island usually begins to subside and the fringing reef turns into a barrier reef. When the island completely subsides beneath the water leaving a ring of growing coral with an open lagoon in its center, it is called an atoll.

Climate change is expected to take a toll on these low-lying islands in the coming years as rising sea levels result in bigger waves and increased flooding. Daily Overview, to commemorate the start of the Paris Climate Conference, also captures an atoll: