Bubbling hot liquid mud, Iceland. In volcanic areas, from deep subterranean cracks in the ground, superheated water and steam take a variety of forms. The steam is laden with chemicals such as carbon dioxide, sulfur dioxide, hydrogen chloride, and hydrogen sulfide. When the acidic hot spring liquifies the muds and clays, great pits can form and the gasses bubbling the mud and filling the air with the sulfurous smell of rotten eggs. The danger lies in the unstable soil that at one time can appear as baked earth and at another a scalding swamp of belching mud. It depends on the water table. Some pools barely bubble and others are explosive, casting hot liquid mud around the pit.