Yellowstone National Park, WY
Mammoth Hot Springs is the main attraction in this neck of the woods. Each day, mineral-laden hot water from deep within the earth pushes to the surface and builds up layers of terraced stone. Up to two tons of new limestone are deposited daily at this sight. Quite a spectacle to see!
When you're done at the springs, put on your bathing suits and jump into the Boiling River. Hot and cold water mix in pools along the Gardner River's edge to create a relaxing bath with large clouds of steam. Daytime bathing only!
Geography trivia: Stand on the 45th parallel! A sign here marks the imaginary line circling the globe between the equator and North Pole. (You'll find it where the north entrance park road meets Gardner River.)