We stopped at Mammoth Hot Springs in Yellowstone National Park and I started to get hungry. Boiling hot water and calcium carbonate produced a smell like hard boiled eggs. Travertine [the white material in this view] is a type of limestone and is formed by a process of rapid precipitation of calcium carbonate, often at the mouth of a hot spring or in a limestone cave. In the latter, it can form stalactites and stalagmites. It is frequently used in Italy and elsewhere as a building material.