Click to view The images you see above are of a deep lake in Camaroom called Nyos — before and after it killed almost 2,000 people with a burst of poison gas. No, this wasn't caused by pollution or global warming. It's a natural phenomenon called a "limnic eruption," and scientists believe that a big one deep beneath the ocean might be enough to cause a mass extinction event.

A limnic eruption is caused when a huge amount of carbon dioxide bubbles up from the bottom of a very deep body of water. Think of it as a kind of volcano, except with poison gas erupting from water, rather than hot liquid erupting from the earth. In fact, they tend to happen in volcanic regions. Limnic eruptions can happen in lakes, as the unfortunate inhabitants around Lake Nyos learned in 1986 when the water emitted so much carbon dioxide that everyone in the vicinity asphixiated. And they can happen in oceans, which are the world's deepest bodies of water as well as being highly volcanic. While limnic eruptions are rare, Environmental Graffiti points out that there is strong evidence that they've happened on a mass scale before:

Carbon dioxide levels in the atmosphere have reached almost eight times the current level on more than one occasion in Earth’s history, leading some scientists to speculate that limnic eruptions have occurred in our oceans before, and may happen again.

For the apocalyptically-inclined, this is one more reason to bring your oxygen tank and respirator everywhere. Nature's Deadly Bong [via Environmental Graffiti]