Hamilton Pool - Hamilton Pool Preserve, Texas
Formed when the roof of an underground river collapsed, Hamilton Pool sits among 232 acres of protected Hill Country habitat, 23 miles west of Austin. The grotto, fed by a 50-foot waterfall that trickles or gushes into the canyon depending on the season, gives the feeling of swimming in a cave with all the lights flicked on. The pool is surrounded by massive limestone boulders, with a portion shaded by a limestone overhang dripping with stalactites. And what about those bat-like creatures swooping and diving in the sky over your head? Don't worry, they're just cliff swallows that nest among the moss and maidenhair ferns on the grotto walls.
More from The Active Times:
12 Surprisingly Spectacular City Beaches »
The National Parks: Ranked! »
The World's Most Surprising Surf Spots »
America's Nastiest Beaches -- and the Cleanest! »