The Hamilton Pool,Austin,Texas,USA

Hamilton Pool Preserve is a natural pool that was created when the dome of an underground river collapsed due to massive erosion thousands of years ago. The pool is located about 23 miles (37 km) west of Austin, Texas off Highway 71. Since the 1960s, Hamilton Pool has been a favorite summer swimming spot for Austin visitors and residents. Hamilton Pool Preserve consists of 232 acres (0.94 km2) of protected natural habitat featuring a jade green pool into which a 50-foot (15 m) waterfall flows. The pool is surrounded by huge slabs of limestone that rest by the water's edge; large stalactites grow from the ceiling high above. The ceiling and surrounding cliffs of the grotto are home to moss, maidenhair fern and cliff swallows. The Ashe juniper (cedar) uplands of the preserve are home to the endangered golden-cheeked warbler.
The natural pool and creek are not chemically treated, so water quality is monitored regularly and swimming is occasionally restricted; call ahead to check conditions before your visit. Hamilton Pool is part of the Balcones Cayonlands Preserve and is a protected environment. There are no lifeguards on duty, and drinking water and concessions are not available. Composting toilets and picnic tables are provided near the parking lot. Since Hamilton Pool is a nature preserve, the following rules apply: no fishing, mountain biking, camping, dogs or other pets. Cooking and fires are also not permitted...

Source:-Wikipedia