Bathers flock to the point where the large hot spring known as Boiling River pours over travertine rock and into the chilly Gardner River. The 140-degree water is thought to flow underground from Mammoth Hot Springs, a couple of miles to the south. The water temperature varies substantially along the 150-foot-long band of pools; position yourself just so and you can freeze your fanny while toasting your toes.
The turnoff to Boiling River is 3/4 of a mile south of the 45th Parallel pullout. Bathing suits are required and no alcoholic beverages are permitted; the pools are closed during the high water of spring (they generally reopen by late June). Watch for bison, elk, and bighorn sheep in the immediate area.