Some UC Berkeley researchers stuck cameras and sensors down some Yellowstone and Chilean geysers to find out.  Here’s what they learned:

The key to geysers, said Michael Manga, a UC Berkeley professor of earth and planetary science, is an underground bend or loop that traps steam and then bubbles it out slowly to heat the water column above until it is just short of boiling. Eventually, the steam bubbles trigger sudden boiling from the top of the column, releasing pressure on the water below and allowing it to boil as well. The column essentially boils from the top downward, spewing water and steam hundreds of feet into the air.

Cool video describing their findings here: