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.
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.