A lake in Cameroon released a gas cloud that killed over 1,700 people in one night

Lake Nyos sits over a pocket of magma that saturates its deep water with carbon dioxide. In August 1986, a disturbance triggered a sudden overturn that released a cloud of CO2 estimated at roughly a cubic kilometer, which rolled down surrounding valleys and asphyxiated people and livestock up to 25 km away as they slept.

— Scientific and geological record, Lake Nyos — Limnic eruption, Cameroon, August 1986