Three miles into Yellowstone from the North Entrance and before you reach Mammoth Hot Springs is a small parking lot on your left. Park and walk down the trail (400m). At the confluence of the Gardiner and Boiling rivers is a fun swimming hole. Mostly known by locals, you'll enjoy the sensation of hot thermal waters from the Boiling River mixing with the icy water of the Gardiner. Take 30 minutes and enjoy.