Blue Hole (Big Pine Key)
The Blue Hole is an attraction on the island of Big Pine Key in the Florida Keys. It is an abandoned rock quarry that was used for nearby road fills and Henry Flagler's Overseas Railroad. The water it contains is mostly fresh and is used by various wildlife in the area, such as birds, snakes, alligators, key deer and green iguanas. It is part of the National Key Deer Refuge.
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.