They may look like something out of the fictional world of Avatar, but these spectacular blue sink holes are far from fantasy.
Formed gradually or suddenly, blue sink holes are natural depressions in the earth caused by the collapse of the surface layer.
Typically located in low-lying coastal regions, their large vertical caves are created by the process of karst the dissolution of soluble rocks such as limestone, dolomite, and gypsum by rain water or streams.
1. The Great Blue Hole, Belize
The Great Blue Hole is a large submarine sink hole off the coast of Belize.
The hole is circular in shape, over 984 ft across and 407 ft deep.
It was formed during several episodes of quaternary glaciation starting over than 150,000 years ago.