An atoll is a ring- or horseshoe-shaped reef of coral enclosing a shallow lagoon. An atoll begins
as a fringing reef surrounding a slowly subsiding island, usually volcanic.
As the island sinks the coral continues to grow upwards until eventually
the island is below sea level and only a ring of coral is left at the surface.
|Tuanake Atoll in French Polynesia – one of
many remote reefs mapped with the help of satellite data. This true-color
image was acquired by Landsat 7. Image courtesy: Serge Andrefouet,
University of South Florida