Coral polyps are animals about the size of your little fingernail. They live in huge groups and build hard limestone cases around themselves. Over thousands of years, millions of polyps build up layer upon layer of reef. When a reef grows up to the surface of the sea, sand collects on top, seeds blow in on the wind, and a new island is born.
Because atolls are formed by coral, they are usually very flat and low - most land in the Marshall Islands is only one or two metres above sea level. But what they lack in height they make up for in colour. The lagoon inside an atoll is often a brilliant turquoise blue, and the reef is full of fish, turtles and rays.
The Marshall Islands has 29 atolls, each one a necklace of small islands around a lagoon. Majuro is the capital atoll and the most populated. Arno Atoll, just south of Majuro, is known for its beautiful beaches and gentle lagoon. Mili Atoll in the south-east has one of the largest lagoons in the whole country.
The coral reef is also a food source, a nursery for young fish, and a natural barrier that protects the land from strong ocean waves. Marshallese people have always respected the reef as a living thing that gives the islands life.