Blue Hole (Red Sea)
The Blue Hole is a diving location on the southeast Sinai, a few kilometres north of Dahab, Egypt on the coast of the Red Sea.
Santa Rosa, New Mexico
Santa Rosa has many natural lakes, an anomaly in the dry desert climate surrounding it. These are sinkholes that form in the limestone bedrock of the area and fill with water, and thus the lakes are connected by a network of underground, water-filled tunnels. The most famous of these is Blue Hole, a popular spot for diving, where cool water forms a lake over deep.