Coal Bed Methane is methane (natural gas) trapped in coal seams underground.
To extract the gas, after drilling into the seam, it is necessary to pump large amounts of water out of the coal seam to lower the pressure. It is often also necessary to frack the seam to extract the gas.
There are a similar catalogue of negative environmental and social effects as with Shale Gas. This includes methane migration, toxic water contamination, air pollution, increased carbon emissions and a general industrialisation of the countryside. Impacts that are specific to CBM include depletion of the water table and potentially subsidence.
In common with other unconventional gas extraction, such as Shale Gas, CBM wells do not produce large amounts of gas per well and production declines very quickly. It is therefore necessary to drill large numbers of wells, covering a huge swaths of the landscape.
CBM exploitation began in the US and over 55,000 CBM wells have been drilled in the last decade or so, mostly in the western states (Colorado, New Mexico and Wyoming in particular).
In Australia, where it is know as Coal Seam Gas (CSG), over 5,000 CBM wells have been drilled in Queensland in the last few years and the industry is aggressively expanding into New South Wales.
In the UK CBM is more advanced than Shale Gas and full scale production may begin soon.