Mammoth Hot Springs mineral terraces. These terraces are composed of travertine, a deposit formed by the precipitation of calcium carbonate. This is deposited because water, heated below ground, emerges from the spring supersaturated with bicarbonates dissolved from the limestone bedrock. At the surface, where pressure lessens, carbon dioxide is released (outgassed) from the water and calcium carbonate is deposited. Photographed in Yellowstone National Park, Wyoming, USA.