34 Supervolcanoes, Most Could Wipe Out Millions When They Erupt

Campi Flegrei (Italy)

Located west of Naples, this 8-mile-wide caldera has shown concerning uplift since the 1950s. The crisis in the 1980s forced thousands to evacuate. Though its last eruption in 1538 created Monte Nuovo, a much larger event 39,000 years ago covered Europe in ash. Recent seismic swarms and increasing gas emissions keep locals and scientists constantly vigilant.