What gas is produced by rotting vegetation?

Methane is a naturally occurring organic compound, but human activity has increased the amount of this potent greenhouse gas that goes into the atmosphere. Most of the methane that humans emit comes from natural gas, landfills, coal mining and manure management, but methane is almost everywhere and it comes from some surprising sources.

The 8,000 hydroelectric dams in the U.S. generate a huge amount of sustainable electricity, but they also produce methane. How? It's all part of the process to create a dam in the first place.

When a dam is built, the area behind the dam is flooded by water that can no longer travel where it used to flow. That leaves a potentially huge amount of vegetable matter — plants and trees that use to exist in the open air — rotting beneath the surface of the water. Rotting vegetation produces methane, and in normal situations that methane would escape into the atmosphere in incremental doses. But the rotting plants behind a dam store up their methane in the mud. When the supply of water lowers behind a dam, all of that stored-up methane can suddenly be released.