Beneath this lake is a deadly amount of methane and carbon dioxide?

Answer 1

You're probably referring to three lakes in Central Africa, Lakes Nyos, Monoun, and Kivu. They periodically undergo what's called "lake overturn," also known scientifically as a limnic eruption. It's a rare set of circumstances: nearby volcanic activity, deep lakes, and an equitorial latitude. The nearby vulcanism means carbon dioxide (CO2) and methane (CH4) bubble up into the lakes' deepest regions. The CO2 dissolves in the water as a result of the increased pressure at that depth (CO2 won't dissolve in water near the surface.) The geographic location of the lakes means there isn't a lot of variance in temperature between seasons, so there's little convection current action going on in the lakes to mix things up. So, the carbon dioxide just accumulates down there, and something like a landslide or small earthquake can shake things up just enough to cause all that CO2 to erupt to the surface. This can cause locally destructive tsunamis, but even more deadly than that, the huge amount of carbon dioxide can blanket whole towns, suffocating people and animals. Carbon dioxide, at 44 grams/mole, is heavier than the mixture of air we breathe, so it doesn't disperse as quickly into the atmosphere. Limnic eruptions in this region have already proven deadly, and efforts are underway to try and "vent" excess CO2 from the lake bottoms before this happens again.