Terry Engelder, a professor of geosciences at Pennsylvania State University who boldly calculated that the Marcellus Shale held 489 Tcf of technically recoverable natural gas before it was transformed into the country's largest gas field, is retiring. While Range Resources Corp. drilled the first horizontal Marcellus well in 2004, Engelder's calculation is credited with attracting the interests of some of the independent oil and gas producers that have pioneered the play. One of his first estimates was more than 25 times that of the U.S. Geological Survey's at the time. He told the university that a question during a 2007 webinar about the amount of recoverable reserves in the Marcellus stumped him. Shortly after, he began crunching numbers. He said the calculation was "by far the highlight of my career." Since then, Engelder has achieved world-renown, granting interviews to nearly 600 journalists worldwide and helping them better understand the geological aspects of the play and the characteristics of the industry that has developed it with high volume hydraulic fracturing.