Lake overturn is a natural phenomenon, where carbon dioxide
trapped in the waters at the bottom of a lake is suddenly released.
Underground springs carry carbon dioxide into the lake. Due to the high pressure of the water at the bottom, the CO2 cannot form bubbles and stays dissolved in the water. When something causes the pressure to drop, bubbles form and rise to the surface, creating a dense cloud of CO2 gas over the lake. This process is the same that is observed when you open a soda bottle.
In recent times, there have been two lake overturns, with catastrophic consequences. In 1984, Lake Monoun, in Cameroon, overturned and killed 37 people by suffocation. In 1986, Lake Nyos, also in Cameroon, overturned. Since the lake is inside a volcanic crater, the cloud of gas filled the crater and spilled down into the surrounding villages, spreading 25km and silently killed 1,800 people and 3,000 cattle. It is estimated that 1km3 of gas was released, lowering the water level of the lake by 1m. The suspect for the trigger of the gas release was a landslide on the side of the lake.
A scheme to pump water from the bottom of Lake Nyos to the surface to release the trapped gas is underway but there are other lakes aroud the world that have the potential to overturn, most significantly, Lake Kivu, in Rwanda. Lake Kiva is one of the largest and deepest lakes in Africa and has 2,000,000 people living around its shores. The area around Lake Kivu is seismically active and an event under the lake could trigger an overturn.