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 popular 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 large 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.