A particularly chaotic type of gaping river hole, a cauldron is characterized by “big, squirrelly, boily water,” in the words of one river runner. River cauldrons form where rocks, which have accumulated on the riverbed at a spot where the river suddenly drops, scour out a bowl-shaped depression. The surface of the river churns and explodes here like soup boiling over in a pot. Sulphur Cauldron, on the upper Yellowstone River in northwestern Wyoming, is one of many famed river cauldrons. Cauldrons are also a seacoast feature. When ocean swells and breakers surge into constricted openings in the seaward face of the land, they sometimes produce ferocious hydraulics—violent whirlpools, geysers, and suddenly collapsing haystacks of white water.