The Homestead Caldera

The Homestead Caldera, known locally as "The Crater" is a natural hot spring surrounded by a rock dome. It is a commercial location open year round to scuba divers and swimmers. Originally, it was accessed from a large natural opening at the top of the dome, but has since had a tunnel blasted through horizontally for easy access. The Crater began forming 10,000 years ago, and is made of minerals deposited by the rising, heated spring water.

http://en.wikipedia.org/wiki/Homestead_caldera