Scientists find thousands of garbage filled craters more than 3,000 feet underwater

Close-up view of the seafloor inside a micro-depression, showing trash, rocks, seafloor animals, and fish. Close-up view of the seafloor inside a micro-depression, showing trash, rocks, seafloor animals, and fish. Photo: MBAI Photo: MBAI Image 1 of / 8 Caption Close Scientists find thousands of garbage filled craters more than 3,000 feet underwater 1 / 8 Back to Gallery

About 20 years ago, scientists were puzzled by the discovery of massive and almost perfectly circular craters, some with diameters more than twice the length of two football fields, on the ocean floor off the California coast.

The Monterey Bay Aquarium Research Institute (MBARI) has more recently counted 5,200 of these so-called "pockmarks" offshore from Big Sur. The 500-square-mile area marks the largest pockmark field in North America.

Using sonar mounted on autonomous underwater vehicles to survey the ocean floor 3,000 to 4,000 feet underwater, the researchers were surprised to also find more than 15,000 micro-depressions (about 11 meters across on average), many filled with debris and garbage.

"Trash, marine debris and dead animals are getting gathered into these holes and concentrated," said Eve Lundsten, a senior research technician with MBARI. "They’re acting like trash bins on the ocean floor."

In total, 30 percent contained manufactured trash and 20 percent marine debris. (See photos above.)

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Researchers are looking closely at this region right now to determine whether it's suitable for a proposed offshore wind farm development. They want to find out whether it's safe to anchor wind turbines into the ocean floor, and mainly whether methane gas that can cause instability is present.

Lundsten, who worked on the research presented at the American Geophysical Union conference in San Francisco in December, says depressions are found throughout the ocean and are sometimes caused by bubbling-up methane gas. But off the Big Sur coastline, Lundsten says there's no evidence to suggest the pockmarks or micro-depressions were created by methane gas.

"None of those special biological communities that feed in methane are here," she said. "The pockmarks remain a mystery. Sonar data showing layers of sea floor sediments suggest they may be 400,000 years old and may have been inactive for the last 50,000 years."

By contrast, the micro-depressions formed in younger sediment and researchers believe local currents may have excavated the soft and fluffy ocean bottom around items — whether a rock, whale skull or barrel — dropped to the ocean floor. A few manmade objects found in the depressions included bags full of trash, 5-gallon buckets and a storage trunk.

"Anything heavy and solid on the sea floor is going to create currents and increase erosion around it," said Lundsten. "But it’s still not clear if they’re getting washed into these micro-depressions. We also can’t rule that out. There’s a lot of uncertainty."

What's more, the team noticed these dimples at the bottom have fostered microhabitats for fish and other creatures that may further contribute to the carving out of the holes.

Amy Graff is a digital editor for SFGATE. Email her at agraff@sfgate.com.