Sonar images reveal an enormous crater at the bottom of the Pacific Ocean.

? 2019 MBARI

What’s creating thousands of craters off the California coast?

SAN FRANCISCO, CALIFORNIA—Just off the coast of California, thousands of craterlike depressions, some as big as buses, dot the sea floor. Now, scientists say they know what’s causing these mysterious features.

Researchers discovered the depressions while investigating a supersize version. In the late 1990s, scientists using ship-mounted sonar found enormous craters—more than 100 meters across and about 5 meters deep—on the bottom of the Pacific Ocean. In most cases, “gas is bubbling up to the sea floor and lifting sediment and leaving a depression,” says Eve Lundsten, a research technician at the Monterey Bay Aquarium Research Institute in Moss Landing, California.

To get a closer look, Lundsten and her collaborators took to the Pacific in 2018 and 2019. They launched underwater vehicles to scan the ocean bottom about 30 kilometers off the coast of Big Sur, a small town south of Monterey.

The new observations revealed a surprise: Roughly 15,000 craters, never before seen, dot the ocean floor near Big Sur, the team reported here today at the annual meeting of the American Geophysical Union. These “microdepressions” are roughly 10 meters across and 1 meter deep—and nearly one-third of them contain garbage. “Pretty consistently, they have things in them,” Lundsten says.

Trash and marine life fill one microdepression off the coast of Big Sur, California.

? 2019 MBARI

This trash is probably indirectly helping create the features, Lundsten says. Garbage attracts marine life, she notes, and the collective swimming and burrowing of creatures like fish, snails, sea anemones, and starfish kicks up fine sediments on the sea floor. Lundsten and her colleagues found marine creatures in many of the trash-laden microdepressions they observed.

Seafloor surveys like these can reveal how humans have affected the ocean, says Reidulv B?e, a marine geologist at the Geological Survey of Norway who was not involved in the study. “In a way, you can map garbage by counting microdepressions.”

Lundsten and her colleagues also found rocks and animal bones in some of the microdepressions—it seems that the trash is creating habitats for marine life, the team concludes. But surprisingly, some of the microdepressions didn’t contain anything at all. It’s a mystery how these empty microdepressions developed, Lundsten says. But they probably aren’t formed in the same way as the giant craters, the researchers propose, because they found no evidence of gas venting.