Hello . I have an inground pool which has been a nightmare since we bought the house last year. It was losing 400 gals a day last summer and I finally gave up on it in the fall. I figured the water would eventually stop dropping where the leak was. I had two pool guys come in, dyentest, pressure test the lines and listen underwater with a mic. So it dropped almost 3 ft over the winter and we could not cover it because of the low water lever. All of a sudden around jan/feb, it stopped leaking and started filling itself back up(from rain/snow). By the time spring hit I had to pump it down. So in late may I hooked everything back up and it was fine for a few day then the leak came back worse. Was losing almost 800 gals a day. So I started ripping the deck up. Found a bad leak around the skimmer gasket to pool and also skimmer was cracked. Replaced all that. Still losing water. I ripped up the dedk over where the pool light was and dug down. Found a big leak. The light housing had a 1/8 hole torn in it from when the light was broken. Plugged that leak . Found a small hole innthe liner(liner is 5 yrs old) and patched it. We think that happened when a cement block fell in pool. Water loss got better but still losing 3-400/day. Did my impersonation of Rambo and crawled under the deck(very creepy. Found snake skins, dead animal bones yuk..) to find a crack in the staircase leaking. Fixed that with some underwater jbweld. Finally the leaks stopped. No water loss for over a weak....now today I discovered a sinkhole on the downward slope from shallow to deep end approx innthe middle. Sinkhole is about 4-5 inches around and approx 2-3” deep. I coild not see any hole innthe liner. I probably lost a total of 6-7000 gals . There are bumpy and rough spots all over under the liner but this is the first sinkhole. I dont think any piping would be under that slope so I suspect its from all the lost water. Any way to fix it without draining the pool(would be over 15000 gals to get halfway down the slope) plus I doubt the liner could be removed and put back in. I was hoping to either live with it or if someone knows another way...I was thinking of poking a small hole in the liner in the middle of the hole and try blasting some spray expanding foam then patch the hole but then after thinking on that , the chemicals in the foam may melt the liner....ideas or thoughts ? It is a 16x32 inground. Can the liner hold in the sinkhole ? It does feel like one side of the sinkhole maybe on a flat rock. I dont think this liner was installed over proper sandy material.