Moles Under the Pool Liner
We have had our above ground pool for 9yrs and never had this problem.
We just replaced the liner for the first time, and now after 3 weeks of being in, we have tunnels in one area.
We are assuming it is a mole or a small snake. There is a pile of sand outside the wall of the pool.
So, now what?
The good thing is the liner is new so draining and re-filling will not be a problem. I would drain the pool, leaving just enough water in it to hold the liner in place and prevent too much shrinking. You would then remove enough top rails to let you pull the liner back out of the way in the area you need to work. One person in the pool can hold the liner back while another person makes repairs to the sand.
While in the pool you might get a better idea as to just what is causing the damage. Getting rid of the critter for good might be the hard part. There is some really stinky bug and critter poison that could be kept around the outside of the pool, this deters a lot of unwanted pests.
Traps might work or even an exterminator if needed. Maybe a neighbor has had similar problems and can offer advice.