A spectacular hot pool in a large travertine basin out in the middle of Monitor Valley, northern Nye County, Nevada, USA. Anywhere else this would be a park, with entrance fees, interpretive stations, and crowds; here it's just out in the BLM land, only shut off by a barbed wire fence! Feature, not bug--
The temperature of the pool is around 160 F, IIRC--not boiling, but plenty hot enough to scald quickly. It obviously is cool enough to support an algal population.
No one seems to know who "Diana" was. Perhaps the name was given by some early settler with a taste for the classics, altho I don't remember the Roman goddess as being particularly associated with things like hot springs. It's also called "Devil's" punchbowl, which is more understandable if a lot more generic.