Accumulations of natural gas may be the cause of a distinctive sea floor feature of some Maine bays. According to a theory proposed by University of Maine geologists Joseph Kelley, Daniel Belknap and students Jeff Rogers of Old Town and Allen Gontz of Orono, gas bubbling from decomposing vegetation may be trapped by mud and other sediments until it is released by natural events or human activities. Earthquakes, sea floor dragging and even anchors dropped from boats may cause bubbles to be released in areas where gas has accumulated in adequate concentrations. While these gases do not appear to be economically recoverable, they may lead to the thousands of small craters or pockmarks that dot parts of the seafloor in Penobscot Bay, Belfast Bay, Casco Bay and other locations. Under most circumstances, sonar can detect bedrock well below the sediments.
Sonar waves can see through the mud, but bubbles of natural gas are impenetrable.
Several theories have been proposed to explain their existence. It has been suggested that whales or flowing groundwater may have been a cause, or even bombs dropped from the surface. However, no evidence has been found to support those suggestions. The pockmarks found by the UMaine team vary in width and depth, from less than 15 feet wide to more than 325 feet wide and up to 100 feet deep. We've observed thousands of these on the sea floor.