he open marine conditions by a barrier (Britannica 2001 Standard Edition CD-ROM). Along the reef, coral islands may form. Fringing reefs may also sometimes form between the barrier reef and the shoreline. These types of reefs lay parallel to the shoreline. Examples of double barriers reefs, where two reefs lay parallel to each other, are very rare, but have been recorded off Mayotte and New Caledonia.
             The last type of reef is the atoll. They are open sea coral reefs and form a ring, which encloses a central lagoon of varying depth. They form when landmasses, such as small islands, disappear below the ocean surface. Low-lying coral islands tend to form on the rim of the atoll due to coral debris that accumulates through wave action. The rim of the atoll may be continuous, which results in the closing off of the lagoon. That, in very few cases, will cause the lagoon to sediment up completely and become covered in palm trees. Like it was stated previously, these cases are very rare. In
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