The Marshall Islands Postal Service issued five new stamps featuring Corals of the Sea.Coral reefs are grouped into three categories: fringing, barrier and atoll. Fringing reefs are submerged living coral platforms that extend from shore into the sea. Barrier reefs run along shore but are separated from land by water. An atoll is a ring-shaped island of coral surrounding a body of water called a lagoon. It is formed when coral builds up on the rim of the crater of a sunken volcano or on a submerged mud bank. Typically, one or more channels connect the lagoon to the open sea.