How were mysterious boreholes on California seabed formed? Methane leak occurred 2,80,000 years ago or sinister mysteries?

What could be the source of thousands of boreholes or pockmarks found on the seabed off Slur in California? Did methane bubbles create these holes like pockmarks found elsewhere? The Monterey Bay Aquarium Research Institute (MBARI) has carried out studies but still needs help finding clear answers to mysteries.