The largest known volcano in the solar system is on Mars, not Earth.

Olympus Mons, a shield volcano on Mars, stands roughly 22 kilometers (about 13.6 miles) high — nearly two and a half times the height of Mount Everest above sea level — and spans a base around 600 kilometers across, an area comparable in size to a small U.S. state.

Olympus Mons grew so enormous partly because Mars lacks plate tectonics. On Earth, the crust shifts over hotspots, spreading volcanic activity across a chain of smaller volcanoes like the Hawaiian Islands. On Mars, the crust stays fixed over the same hotspot for hundreds of millions of years, letting lava pile up in the same place indefinitely.