Mammoth Hot Springs, once known as white mountain, looks more like the inside of a cave than a hot water fountain. What gives this hydrothermal area its distinctive appearance? A unique combination of rock and water. The rock below Mammoth Hot Springs is limestone rather than the silica-rich lava flows found elsewhere in Yellowstone. What's more, the superheated water at Mammoth contains a gas called carbon dioxide that dissolves limestone as it percolates up. Calcium carbonate, the principle mineral in limestone, is carried to the surface by the hot bubbling water. When the water reaches the surface it changes pressure and cools. The cooler water releases the gas and deposits the calcium carbonate mineral. The deposits are layered one on top of another forming travertine terraces one step at a time.