The Furnace Below

Under Yellowstone lies a colossal magma chamber, tens of kilometers across and several deep. Most of it is solid or pasty rock, with only a fraction hot enough to flow — one reason an eruption is not imminent.

This hidden heat drives the surface shows. Old Faithful erupts because groundwater seeps down, touches scorching rock, flashes to steam, and rockets back up — a pressure cooker venting on a near-clockwork schedule.