The Great Blue Hole, located near Ambergris Caye, Belize (Credit: U.S. Geological Survey /Public domain)

Scattered across the ocean bed, and often hidden from the human eye, are hundreds, or perhaps even thousands, of "blue holes." The massive underwater sinkholes, which host a diverse biological community — ranging from coral to sponges to sharks to sea turtles — were formed thousands of years ago when groundwater dissolved karst, a type of porous limestone rock found on ocean floors.