Coal, another fossil fuel, is formed underground under similar geologic conditions as natural gas and oil. These coal deposits are commonly found as seams that run underground, and are mined by delving into the seam and removing the coal. Many coal seams also contain natural gas, either within the seam itself or the surrounding rock. This coalbed methane is trapped underground, and is generally not released into the atmosphere until coal mining activities release it. Historically, coalbed methane has been considered a nuisance in the coal mining industry. Once a mine is in place, and coal is extracted, the methane contained in the seam usually leaks out into the coal mine itself. This poses a safety threat, as too high a concentration of methane in the mine creates dangerous conditions for coal miners. In the past, the methane that accumulated in a coal mine was intentionally vented into the atmosphere. Today, however, coalbed methane has become a popular unconventional form of natural gas. This methane can be extracted and injected into natural gas pipelines for resale, used as an industrial feedstock, or used for heating and electricity generation. The Potential Gas Committee estimates that 163.0 Tcf of technically recoverable coalbed methane exists in teh United States, making up 7.8% of the total natural gas resource base.

What was once a dangerous by-product of the coal industry is becoming an increasingly important source of natural gas. 

Quoted from: http://www.naturalgas.org/overview/unconvent_ng_resources.asp