Katla Caldera (Iceland)
Concealed beneath the Mýrdalsjökull glacier, Katla’s last eruption in 1918 broke through ice hundreds of feet thick. Scientists believe a major event is overdue. When eruption happens, rapidly melting ice creates catastrophic floods called jökulhlaups. Steam rises through cracks in the glacier, evidence of the heat below. Nearby residents maintain evacuation plans for when—not if—it awakens.