In grade school everyone has their own theory about the extinction of the dinosaurs: the climate changed, they were too big for their own good, they turned into birds or an asteroid wiped them out. But plenty of mass extinctions have yet to be explained.

Gregory Ryskin, a Northwestern professor of chemical engineering, decided to take on the mother of them all — an extinction that killed 96 percent of marine life at the end of the Permian period, 248 million years ago. More than 75 percent of vertebrate land species also were lost.

And no one agrees on what happened.

Ryskin, who has no formal training in geology or paleontology, spent years reading up on the field before he proposed his theory. He suggests that methane-saturated water deep within the ocean could have burst into the atmosphere, igniting explosions more powerful than all the planet’s nuclear weapons combined.

“Compared to it all the global warming problems would be child’s play,” Ryskin said.

Methane could have accumulated in the ocean depths through the decay of bacteria or from frozen methane hydrates near the ocean floor, Ryskin contends. A small disturbance, such as a meteorite or a tremor, could push the methane-saturated water close enough to the atmosphere to explode.

Though his theory has yet to gain acceptance in the field, Ryskin argues it is the only explanation that accounts for every aspect of the most massive loss of life on Earth.

Ryskin began to think in 1986 about how an eruption from the sea could cause mass extinctions. That year an accumulation of carbon dioxide erupted from Lake Nyos in Cameroon, sending a huge plume of water into the air and suffocating more than 1,700 people.

Ryskin began to wonder if the same thing could happen on a global scale. He didn’t act on his idea right away, though.

“For a couple of years, I didn’t do anything,” he said. “For a chemical engineer to start working in biology is a big jump.”

So Ryskin hit the books. He estimates that over the next few years he read hundreds of volumes and about 20,000 scientific papers on geology, paleontology and oceanography.

“He’s educated himself quite a bit,” said Brad Sageman, an NU geological sciences professor. “He also talked to every scientist I know in the Chicago area who knows everything about this.”

Ryskin originally was sure someone already had thought of his theory. But when he found that no one had, he began to work on a paper laying out his ideas. After numerous drafts he circulated it among experts, looking for constructive criticism. But only about one in eight provided useful comments, he said.

“Thank God professors have some freedom of research,” he said. “There was no chance of getting any grants for this research. Trying would have been totally useless.”

After being rejected three times, the paper finally was published in the September issue of Geology magazine. Ryskin said he was elated.

“I would never give it up, but I was never sure it would be published,” he said.

Sageman, who was one of the experts Ryskin consulted, said that while he has an interesting idea, it will be very difficult to test for the presence of ancient methane deposits.

David Archer, a professor of oceanography and geophysics at the University of Chicago, said he doubts any part of the ocean would be stagnant long enough for the methane to build up.

Archer, whom Ryskin acknowledges in his paper, said he respects his colleague’s efforts to integrate scientific disciplines.

“The potential is there for somebody outside of a field to really come up with some new ideas that stodgy people like me who are in the field all the time might overlook because they’re too wild,” he said. “If his theory was widely accepted, this would be a very big deal.”

Ryskin said his geological work will soon be finished, but he hopes geologists doing field work will further investigate his ideas.