The Marcellus natural gas trend is a large and prolific area of shale gas extraction from the Marcellus Formation of Devonian age in the eastern United States. The trend encompasses 104,000 square miles and stretches across Pennsylvania and West Virginia, and into southeast Ohio and upstate New York. It is the largest source of natural gas in the United States, and production was still growing rapidly in 2013. The natural gas is trapped in low-permeability shale, and requires the well completion method of hydraulic fracturing to allow the gas to flow to the well bore. The surge in drilling activity in the Marcellus Shale since 2008 has generated both economic benefits and considerable controversy.