A ring in the ocean
An atoll is a ring-shaped coral reef that surrounds, or nearly surrounds, a lagoon. The lagoon is the water inside; the reef forms its boundary.
From above, an atoll can look like a necklace dropped into the sea. Some beads are sandy islands. Others are sections of reef beneath the water.
Not a solid hoop of land
NOAA describes atolls with islands and gaps in the reef. You do not need one continuous strip of dry land to make the ring.
The classic explanation starts with coral growing around a volcanic island. As the island slowly sinks and the reef grows upwards, a lagoon separates the reef from the land. Eventually, the island can disappear below sea level.
The full story has more moving parts
That is a useful starting picture, rather than a complete recipe for every atoll. Changing sea levels, reef growth, sediment and the dissolving of carbonate rock also shape the result.
USGS research using Enewetak Atoll and computer models explores how those processes help create the raised reef rim. Nature rarely sticks to one neat sketch.
Atoll or lagoon? Look at what you mean
A lagoon is a body of water partly separated from a larger one by a natural barrier. Coastal lagoons exist too, so a lagoon does not automatically mean an atoll.
For two other landscapes worth a closer look, meet a valley with the sea inside and discover where the ocean gets its salt.