PORTLAND, Maine - The Maine coast has dozens of methane gas fields on the ocean bottom where mud-trapped gas occasionally bubbles to the surface, according to a team of University of Maine scientists.

There are 70 known gas fields between Portland and Eastport, and the rising bubbles create craters or pits, according to the scientists, who are publishing their findings in Marine Geology magazine. The largest crater is the size of a football stadium.

The gas fields have no commercial value, the scientists say, but they could pose a hazard for man-made objects on the ocean floor such as utility lines that connect the mainland to Maine's islands.

The scientific team is led by geologists Joe Kelley and Daniel Belknap, who say fishermen over the years have reported seeing bubbles and plumes of mud, and divers have told stories of craters that produced bubbles like carbonated soda.

The ocean floor off the Maine coast, Belknap said, is surprisingly active.

"I visualize a pot of tomato soup bubbling constantly," he said.

Most of the craters are between 32 feet and 260 feet in diameter. The largest, in Belfast Bay, is more than 650 feet wide and 100 feet deep.

In their paper, Kelley and Belknap argue that the craters are evidence of gas eruptions, with gas seeping out of the soupy mud through sandy veins and burping through the ocean floor with enough violence to create bowl-shaped pockmarks.

Three-dimensional sonar images of the bay's floor reveal there are thousands of pockmarks grouped together, like dimples on a golf ball. Similar crater fields are found off Nova Scotia and Labrador, the Pacific Northwest, Alaska and the North Sea.

But some dispute the findings.

Charles Paull, lead scientist at the Monterey Bay Aquarium Research Institute in California, said he has yet to see evidence that the craters are produced by gas.

Paull probed the pockmarks in Belfast Bay several years ago, but found nothing unusual, he said. He found more methane in the Penobscot River than in the bay itself.

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

Kelley said it is difficult to find evidence of the gas in a single attempt because the gas is emitted only occasionally. Because the gas is trapped in mud, it would cost too much to extract it for commercial energy purposes.

But core samples of the pockmarks have contained significant quantities of methane, he said, and scientists have spotted muddy plumes in the water and detected eruptions using sonar.