I have a 1/3 acre farm pond that was dug 18 months ago . I have noticed small...
Many animals burrow. Some will commonly self colonize ponds, but may take a little time to find a new pond. My first guess in your case (in the absence of other evidence) is crayfish. Some crayfish species that burrow into submerged substrates will have complex burrows that connect to openings on land, and some not.