Gas trapped in the mud off the Maine coast occasionally bubbles to the surface, creating craters on the sea floor as large as the Rose Bowl, according to scientists at the University of Maine.

Between Portland and Eastport, there are 70 known methane gas fields, primarily in the deepest and muddiest bays close to shore. Although the gas fields have no commercial value, the scientists say, some pose a hazard for man-made objects placed on the sea floor, such as utility lines that stretch between the mainland and islands.

The team of scientists, led by UMaine geologists Joe Kelley and Daniel Belknap, are publishing their findings in the journal Marine Geology.

For decades, scientists have been puzzled about the pockmarks. Most are between 32 and 260 feet in diameter. The largest, located in Belfast Bay, is more than 650 feet wide and 100 feet deep.

In their paper, Kelley and Belknap argue that the pockmarks are evidence of gas eruptions.

Trapped in soupy mud, the gas occasionally seeps through sandy veins and burps through the sea floor with enough violence to create pockmarks that are shaped like cereal bowls.

The largest collection is found in Belfast Bay. Three- dimensional sonar images of the bay's floor reveal there are thousands of pockmarks grouped together, like the dimples on a golf ball.

Similar pockmark fields are found off Nova Scotia and Labrador, as well as the Pacific Northwest, Alaska and the North Sea.

Kelley said Maine fishermen in the past have reported seeing bubbles and plumes of mud. Divers have told stories of pockmarks that produced bubbles like the carbonation in soda. During the Cold War, a fisherman in Belfast Bay saw bubbles and concluded that a Soviet submarine was prowling the Maine coast.

The ocean floor off the Maine coast is surprisingly active, Belknap said. "I visualize a pot of tomato soup bubbling constantly," he said.

But some dispute the findings. Charles Paull, lead scientist at the Monterey Bay Aquarium Research Institute in California, said his research in Maine has yet to turn up evidence that the craters are produced by gas.

Several years ago, Paull came to Belfast Bay to find out how gas was affecting the environment. But he detected more methane in the Penobscot River than in the bay itself. He also probed the pockmarks for gas, but found nothing unusual.

"It turns out these pockmarks have been about the dullest places I have ever gone to study," he said.

Kelley said the gas is emitted only occasionally, making it difficult to find evidence in just one attempt. He said core samples of the pockmarks retrieved significant quantities of methane. Also, Kelley and colleagues have detected eruptions using sonar and have spotted muddy plumes in the water.

Because the gas is trapped in mud, it would be too expensive to use it as a source of energy, he said.

The pockmarks are recent phenomena that began forming after glaciers began retreating from the Gulf of Maine some 11,000 years ago, Kelley said.

At the time, the world sea level was much lower than it is today, and Maine's coast was many miles seaward of the modern shore. Two common features in Maine's current landscape must have also existed in the past as well: lakes and wetlands, Kelley said.

As the glaciers melted and sea level rose, the lakes and wetlands became submerged. Bacteria consumed the dead plants and animals produced methane gas.

In the sandy areas in the most southern part of Maine, the gas simply bubbled away as it was produced, he said. But in the muddy areas, the gas is trapped until its pressure builds up. In some places, the gas migrates along sandy layers of sediment and erupts in linear chains, he said. In other places, the gas erupts out of the muddy sediment and forms large craters.

With increasing activity offshore, the gas fields are a potential hazard, Kelley said.

He recalled an Army Corps of Engineers proposal to dispose of dredged material at Sears Island by filling in the holes in Belfast Bay. That would have been disastrous, Kelley said, because dropping tons of rock and mud on the sea floor would have caused large gas eruptions.

He said utility companies need to be careful where they position their underwater lines because a gas eruption could damage them.

The three current proposals to build liquefied natural gas terminals in Passamaquoddy Bay would face the same issue, he said. Structures, such as a long pier or a pipeline, could still be built on the sea floor, but they would have to be carefully positioned to avoid any gas fields, Kelley said.

Kelley said more studies are needed to map the sea floor along the Maine coast. Casco Bay, he said, should have a lot of pockmarks, but the bay has never been examined thoroughly. He said there are gas fields between Chebeague and Cousins islands and on the eastern side of Peaks Island, but there are no pockmarks, perhaps because they have been obliterated by the bay's fast-moving currents.

For years, Kelley said, he would turn off his sonar equipment when traveling over mud because he thought mud never changed.

"I thought there was nothing going on," he said. "Then you find it's the most exciting place there is."

Staff Writer Tom Bell can be contacted at 791-6369 or at: tbell@pressherald.com