Tufas, South Tufa area, Mono Lake, California, USA. Tufas are formed when underwater springs rich in calcium mix with lakewater rich in carbonates. As a result calcium carbonate-limestone is formed around the spring. In time the deposits grow into rocks and towers. This process only takes place underwater. The tufas at Mono Lake are exposed because water levels dropped after 1941 when water diversions began. Water levels were allowed to rise again since 1994, but they are still about 2 m (9 ft) below the goal.