34 Supervolcanoes, Most Could Wipe Out Millions When They Erupt

Yellowstone Caldera (USA)

This massive supervolcano spans Wyoming, Montana, and Idaho, with a caldera stretching 43 by 28 miles. Its last major eruption 640,000 years ago ejected 240 cubic miles of ash. Now, it powers the famous Old Faithful geyser while scientists monitor frequent earthquakes. The ground swelling indicates magma movement below, a silent warning of the sleeping giant’s potential.