could be ground hogs, moles or voles...........if you have grubs in your ground it could be moles, we have 3 cats on patrol...........they take care of the moles & voles.....they also killed a couple of very young ground hogs. You could try dumping white vinegar and black pepper down the hole............and fill it with stones......and then put the sand back............you have to find all the openings. they usually have quite a few ways out of the ground. I did see a solar unit in one of those catalogs that come to the house....they also have a detector that suddenly gives out a spray of water.