geyser

A hot spring that periodically erupts, shooting boiling water and steam into the air — powered by a specific plumbing geometry in which superheated water in a narrow underground channel reaches a pressure threshold and flashes to steam, expelling the column above it. Geysers are rare: only about a thousand exist worldwide, most in Yellowstone, Iceland, and New Zealand. Old Faithful is the archetype, but its faithfulness is the exception — most geysers erupt irregularly.