Holes made by a clam gun
If you often go to beaches for the summer, you’ve probably seen these perfect circles dug into the sand. No, these aren’t alien writing, though that’d be pretty awesome. These are actually the leftover marks of a clam gun. People who hunt for geoducks or razor clams use clam guns to remove them from their sandy dwellings.
However, most clam guns are only about 4 to 6 inches in diameter. These marks must’ve been left over by a professional clam hunter, seeing how massive they are compared to the OP’s sandal. And to think that they stuck the gun 4 feet into sand while covering such a wide area is bonkers.