What is a geyser?

A geyser is a rare type of hot spring where groundwater seeps deep into narrow underground channels and is heated by hot volcanic rock. Because the water at the bottom is under high pressure from the water above, it can become superheated well above its normal boiling point without turning to steam. Eventually some of it flashes into steam, which forces the water above it upward and out, releasing the pressure and triggering a violent eruption of scalding water and steam. After erupting, the channels refill and the cycle repeats. Geysers require a special combination of heat, water, and plumbing, so they are found in only a handful of places, most famously Yellowstone in the United States, Iceland, and New Zealand.