A limnic eruption is one of the rarest occurring natural disasters which can be best compared to popping open the largest, most pressurised soda can of all time. When a large gas bubble of CO2 trapped at the bottom of a lake explodes it is called a limnic eruption or a lake overturn.
This occurs in water bodies with cold, high pressure bottoms which are close to a CO2 source like a volcano. The magma releases huge volumes of CO2 which can filter through the earth's crust into the bottom of the lakes. This gas gets dissolved and built up due to the ideal conditions of cold temperatures and high pressures. The saturated gas makes the lake very unstable but a trigger like an earthquake, landslide or rain storm can set off the eruption. The trigger pushes this gas-saturated water higher in the lake where the pressure isn't sufficient to keep the gas in the solution as a result of which it explodes.
This whole process is very rare because in most of the lakes when temperatures change, the water circulates and releases the gas. But in lakes like Nyos and Monoun in Africa where the layers of the water don't mix, the gas keeps on accumulating forming a giant bubble.