The Marcellus Shale formation can be found beneath about 60 percent of Pennsylvania's total land mass, where it is buried to depths of up to 9,000 feet. The most prospective areas for natural gas production are where the shale is present at least 5,000 feet below ground surface. The thickness of the shale formation in regions where drilling may be feasible range between 50-250 feet. Geologists use a number of tools to determine where the formation may have the best potential to produce natural gas, and how to best extract it. Named for a distinctive outcrop near the village of Marcellus, New York in the United States, it extends throughout much of the Appalachian Basin.