Atolls are geological formations made up of :

An atoll is a ring-shaped coral reef, island, or series of islets. An atoll surrounds a body of water called a lagoon. ... The type of corals that build reefs are called hermatypic corals, or hard corals. Hermatypic corals create a hard exoskeleton of limestone (calcium carbonate).

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tropical storm katrina, and it made landfall between miami and fort lauder-dale, florida, as a category 1 hurricane (a storm that, on the saffir-simpson scale, exhibits winds in the range of 74–95 miles per hour [119–154 km per hour]). sustained winds of 70 miles per hour (115 km per hour) lashed the florida peninsula, and rainfall totals of 5 inches (13 cm) were reported in some areas. the storm spent less than eight hours over land. it quickly intensified when it reached the warm waters of the gulf of mexico.

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