On the seafloor off the coast of California, more than 5,000 pockmarks, (some of them being more than 100m diameter!) and tens of thousands of other micro depressions were identified decades ago, but their origin still remains a mystery to the scientific world, with all the possible explanations coming short. Well, except the one explanation for all things beyond science; maybe the holes are a result of a past alien encounter! Maybe(s) aside, this subject will remain a mystery until a foolproof explanation can be brought forward.

More information:

https://www.mbari.org/massive-holes-in-seafloor/