Shale Gas

Interesting video explaining the process of shale exploration here.

Shale gas has become an increasingly more important source of natural gas in the United States over the past decade. ... Although shale gas has been produced for more than 100 years in the Appalachian Basin and the Illinois Basin of the United States, the wells were often economically marginal. Higher natural gas prices in recent years and advances in hydraulic fracturing and horizontal completions have made shale gas wells more profitable. Shale gas tends to cost more to produce than gas from conventional wells, because of the expense of massive hydraulic fracturing treatments required to produce shale gas, and of horizontal drilling. However, this is often offset by the low risk of shale gas wells.

From Wikipedia.