Answer: Carbon Dioxide

If you’re paranoid about dying in an extremely horrible fashion that has an extremely remote chance of ever happening, we’d advise you to stop reading right now. If you’re morbidly curious about such matters, then by all means continue.

Among all the ways one can be done in by Mother Nature, a limnic eruption is perhaps the most startling in both how unexpected and stealthy it is. A limnic eruption, also called a lake overturn, occurs when a deeply held reserve of dissolved carbon dioxide bubbles up suddenly and catastrophically from the depths of a lake and forms a gas cloud like vapors erupting from a witch’s cauldron. The erupting carbon dioxide quickly displaces regular atmospheric gases and blankets the entire area surrounding the lake in a thick and, because carbon dioxide is clear and odorless, undetectable cloud of asphyxiating gas.

The phenomenon only occurs in specific, limnically active lakes (the risk of your average local fishing hole doing you in with a limnic eruption is non-existent) where a set of geological conditions conspire to slowly stockpile the lowest depths of the lake with carbon dioxide rich cold water. Eventually, the saturation point is reached and anything that destabilizes the mixture—earthquakes, nearby volcanic eruptions, even heavy storms or landslides—triggers the release of the gas.

The largest such eruption in recorded history happened at Lake Nyos, in Cameroon. The 1986 eruption there expelled over 80 million cubic meters of carbon dioxide in a dense cloud that killed around 1,700 people and 3,000 livestock animals.