Natural hot springs!
This heated water comes from the ground's crust. People have always gathered around and taken advantage of these oases since before the Vikings.
Clothing is optional. NOT in Utah, but always be careful if you're bringing the kiddles along anyway!
I've passed several natural hot springs here in my state, but have never had the true desire to dip into something that might house single-celled organisms. *Shiver*
Have you been to a natural hot spring? Share your experience!