Firehole Canyon, above the confluence of the Firehole and Gibbon rivers, is home to rapids, flats, cascades and pools. And it is home to the 40-foot Firehole Falls and exposed lava flows 800 feet thick. Yet the real reason for this 2-mile detour is the swimming hole. It is one of only two safe, and legal, swims in Yellowstone! (Boiling River, near Mammoth Hot Springs, is the other.)