Limnic eruptions 

Deep lakes in volcanic areas can be dangerous when carbon dioxide accumulates in the cooler water near the bottom. As it is known carbon dioxide content increases in higher solubility cold water. Therefore, the lower layers of the lake may contain higher levels of CO2. In tropical areas when the surface water is much warmer there may be a strong temperature gradient between the upper and lower layers. In those cases, the temperature difference may generate unstable conditions. Any perturbation, such as a mild earthquake, arrival of warmer fresh water and even a change of atmospheric pressure may produce a sudden liberation of the accumulated carbon dioxide which will rise to the surface The liberated CO2 will form a layer on the lake surface and shores affecting the coastal population. Because CO2 is heavier than the air it will remain forming a layer on the lake surface and shores affecting the local population. Although CO2 is not a toxic gas it cannot be breathed and, when the content of oxygen becomes too low people or CO2 too high, they may die of asphycie. This phenomenon is calle limnic eruption.

This is what happened in lake Nyos in Camerron. In this case the event resulted in the supersaturated deep water rapidly mixing with the upper layers of the lake, where the reduced pressure allowed the stored CO2 to effervesce out of solution.

It is believed that about 1.2 cubic kilometres of gas was released. The normally blue waters of the lake turned a deep red after the outgassing, due to iron-rich water from the deep rising to the surface and being oxidised in the air. The level of the lake dropped by about a meter and trees near the lake were knocked down.  As a result of this gas eruption 1746 people wer killed, as well as 3500 livestock.

Scientists concluded from evidence that a 100 m column of water and foam formed at the surface of the lake, spawning a wave of at least 25 metres that swept the shore on one side.

Carbon dioxide, being about 1.5 times as dense as air, caused the cloud to "hug" the ground and move down the valleys, where there were various villages. The mass was about 50 meters thick, and travelled downward at 20–50 kilometres per hour (12–31 mph). For roughly 23 kilometres the gas cloud was concentrated enough to suffocate many people in their sleep in the villages of Nyos, Kam, Cha, and Subum.[3] About 4,000 inhabitants fled the area, and many of these developed respiratory problems, lesions and paralysis as a result of the gas cloud.

Another event of limnic eruption took place in lake Mounom, also in Cameroon. Because it is a smaller lake and its shores are less populated there were less victims (37 people died). 

This type of phenomenon is very rare and seems to happen mainly in volcanic areas where there are CO2 emanations.