I don't think it is moles. We do have a mole in our front yard, but his trails are a good 8 in wide. The canyons are maybe an inch wide and an inch deep. The don't run the hole diameter of the pool. My guess is that it is ground cracks - or maybe the sand wasn't packed well under the liner. It really isn't causing a huge problem - just more work when cleaning it. We try not to stand too much on the bottom of the pool anyway. Well, it is closed for the season anyway so we will see what it looks like come spring.