Methane

Methane was a chemical compound. Kroll produced methane during its long periods of dormancy on Delta III. This was harvested by a refinery set up by humans. Such large quantities were gathered that the native Swampies' rights were ignored in order to gather it. (TV: The Power of Kroll)

According to Rose Tyler, cows farting methane into the atmosphere was one of the causes of global warming on Earth. (AUDIO: The Flood)

The Vervoids could produce marsh gas, a methane derivative. (TV: Terror of the Vervoids)

The Eye of Gaar was made of ammonia and methane ice. (PROSE: Hot Ice)

The Thara relied on methane to survive. (PROSE: The Roots of Evil)

Waste buried under a hotel owned by Jack Robertson produced methane, sulphides, and trichloroethylene. That caused mutation of spiders, allowing them to grow larger than normal. (TV: Arachnids in the UK)