A Cameroon lake once suffocated 1,700 people while they slept

In 1986, Lake Nyos in Cameroon suddenly released a cloud of magmatic carbon dioxide that had been silently accumulating in its depths for centuries. Being heavier than air, the invisible gas rolled downhill through valleys and asphyxiated roughly 1,746 people and thousands of livestock within 25 km. It is one of only a handful of lakes on Earth known capable of this 'limnic eruption,' and since 2001 engineers have run permanent degassing pipes to bleed off the CO2 safely.

— Wikipedia contributors, Lake Nyos disaster — Wikipedia; event of August 21, 1986