Why holes at the bottom of the ocean disappear and reappear

In Puget Sound, there are areas where gray whales feed on buried prey in tidal zones specifically the beach at Langley) where they basically wind up liquefying the sand, sucking out the food, and letting it all settle back down.

Then, when the tide is out, there are these slightly depressed poorly consolidated areas that are effectively quicksand -- you can go from hard sand to knee deep in a few feet/seconds.

Nice illustration of the scientific process that's not so abstract that even other Biologists wouldn't understand, as is often the case in other fields. Very cool!

Some types of pufferfish are known to build complex circular nests to attract mates and protect their eggs: https://www.youtube.com/watch?v=YOGvVn7IWVY

And a lovely song about it: https://moulettes.bandcamp.com/track/pufferfish-love

What a fascinating reason.

Bottlenose dolphins can do this also. They definitely eat eels.

Purposely made!

TL;DR : Eels hid beneath the sand, porpoise drags them out creating a 'hole'