Hot mud pool at Namaskard, Iceland

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