Barrier reefs usually form between 2,624 and 3,280ft (800m-1km) from the coastline, leaving the currents to dig a lagoon inbetween. While the coral grows to form a more or less continuous reef crown encircling the island, the lagoon accumulates sand deposits which form shoals. They are often dotted with coral 'potatoes' visible on this aerial photo thanks to the transparency of the water. These potatoes are formed of slow growth massive species adapted to this particular environment.