The Great Blue Hole, located just 100 kilometers (60 miles) off the coast of Belize, is an underwater sinkhole that researchers believe is the largest of its kind. Circular in shape and characterized by its rich, blue color, it is over 300 meters (984 ft) across and 125 meters (410 ft) deep. It lies in the center of an atoll called Lighthouse Reef, where an island of coral encircles the shallow, light turquoise-colored waters of a lagoon. Water levels there are so shallow that parts of the ring surrounding the dark blue sinkhole are even known to crest the surface at low tide.

The sinkhole originally formed as a limestone cave during the last glacial period, a time when sea levels were much lower. As the ocean began to rise, the cave system flooded and eventually collapsed, creating a “vertical cave” in the ocean. As such, the site is popular among divers, who flock to the area to see the geological formations that now lie in the ocean’s depths. (x)