Pennsylvania State University geoscientist Terry Engelder spent most of his career toiling in obscurity, studying fracture behavior of rocks known as black shales. Even among geologists, he says, it was kind of a boring topic, and he was often slotted to present his papers on the last day of professional conferences.

"Not only was it the last day, but it was in the afternoon of the last day," he said.

But then, Engelder and Gary G. Lash, a colleague from New York, discovered natural gas in the Marcellus Shale. Rather, they calculated the huge amount of natural gas contained in the shale and announced it to the world in early 2008.

And the world discovered Terry Engelder.

With the emergence of the Marcellus Shale as one of the world's premier natural-gas reserves, the 67-year-old Engelder has become widely identified as a leading advocate of shale-gas development. Now, he travels the world to attend conferences - and often is a keynote speaker.

As a public evangelist for hydraulic fracturing, the controversial process used to crack shales to release natural gas, Engelder has become a target for anti-drilling activists who regard the fossil-fuel rush as an environmental disaster.

One critic of Penn State's role in natural-gas development called Engelder "the poster child for that corporatized institution we used to call a university." Engelder jokes that he is regarded as the "Dr. Strangelove of the fracking debate."

The criticism doesn't bother him, Engelder says. But Anthony R. Ingraffea, a Cornell University engineering professor and anti-drilling advocate who has known him for more than 30 years and has debated him three times in public about fracking, said Engelder has a thin skin.

"Terry's a gregarious, open, friendly, high-energy person," Ingraffea said. "I guess it just comes as a shock to him when he doesn't get the same reaction from other people."

Ralph Kisberg, a founding member of the Responsible Drilling Alliance in Williamsport with whom Engelder has clashed, regards the Penn State professor warily, saying, "There's something a little off there."

Engelder is quirky. He sprinkles his speech with expressions like by golly and gosh and whatnot.

He keeps spreadsheets listing all the people he has spoken with since the Marcellus boom took off - scores of investors, lawyers, engineers and analysts, and more than 450 journalists. He has made more than 300 public appearances. His collection of business cards fills nine boxes on his desktop. They are, of course, organized.

Before Engelder began teaching at Penn State in 1985 - a native of western New York, he has degrees from Penn State, Yale, and Texas A&M Universities - he started jotting down his activities in notebooks. He now famously passes around the notebooks at public appearances for people to sign, so he has a record of the lives he has touched. He is currently on Book #42.

Some regard the notebooks with bemusement. But the only time people refused to sign was at a 2010 City Council hearing in Philadelphia on Marcellus development.

"People said, 'You brought all this calamity on Pennsylvania. We're not going to sign your book.' That was nasty," he said.

Engelder is keeping a record, he says, because he believes he is playing a pivotal role in an American energy revolution, moving away from coal and oil to cleaner-burning natural gas. He also says the nation will become more reliant on renewable energy in the long run, but his convictions about shale gas are so ardent many people don't hear the green-energy argument.

"This is historic," he said of shale gas, an assessment his intellectual adversary Ingraffea calls "grandiose."

Engelder argues that the benefits of natural-gas development outweigh the risks. But he acknowledges there are risks and says some people will make sacrifices for the greater good of producing a vital energy source. His comments have prompted some anti-drilling activists to declare areas "no sacrifice zones."

To those who have called for moratoriums on fracking, he argues the industry is rapidly improving its practices, just as the developers of aircraft and automobiles did with successive generations.

"If you don't do it at all, you don't ever learn how to do it," he said.

After Penn State trumpeted Engelder and Lash's findings in 2008, the professor was summoned to Harrisburg to meet with state policy makers. In 2011, Foreign Policy magazine named Engelder, Lash and shale-gas pioneer George Mitchell to its list of Top 100 Global Thinkers - ranked higher than many heads of state, Engelder notes.

"I don't think many people have experienced the transition from being an unknown university professor to somebody in the public eye," he said.

Lash, Engelder's co-author and a structural geologist at the State University of New York at Fredonia, says he was happy to let Engelder take on the public advocacy role as the shale-gas debate became more polarized.

"I really enjoy just talking about the science," Lash said.

"Terry's very passionate that natural gas is about to change the nature of energy for the next few decades," he said. "He's the guy you want out there doing that."

Sometimes, Engelder comes across as immodest. A radio interviewer called him out for comparing his work to that of Louis Pasteur and Jonas Salk. In discussing his willingness to talk to journalists, Engelder says Albert Einstein was also media-friendly - it's part of the mission of selling America on natural gas.

"I wouldn't be well known now if I wasn't accepting of these things that come along with it," he said.

The notoriety has its benefits. Engelder and Lash are attracting more funding for their research on Devonian shales, the rock in which oil and gas formed over hundreds of millions of years from accumulated dead microscopic sea life.

Engelder also has acquired star status among students. Enrollment in his advanced geology class for engineering students has ballooned from 20 to 120 in five years, said Jon Schueth, his teaching assistant. Engelder drops lots of names in the first lecture to let his students know his place in history.

"It makes us respect him on the first day, how he really is a great thinker," said Patrick Lambert, an engineering senior from Suquehanna County.

But Engelder is sensitive. His prestige is deeply invested in the success of the Marcellus Shale. He spent much of a recent lecture debunking critics who say the country is experiencing a shale-gas bubble -- that "there's not as much gas as this crazy professor at Penn State says there is."

If anything, he says, the enormous Marcellus production is showing that his initial projections were too conservative.