I have a 30,000 gallon in-ground vinyl liner pool. We bought the house 2 years ago, and the pool is about 20 years old. I don't know anything about its construction. Just this year we noticed a depression in the bottom of the shallow end- about 8" in diameter. Applying moderate pressure, the liner depresses, but I can't feel a solid bottom. This area is about 2' from the side of the pool, and there is a large concrete deck all around the pool, 10' wide. I have been told by someone that here in Mississippi under the liner is probably sand, not concrete. Could this be true? Not sure, but the hole may be getting bigger. There does not appear to be a leak now, but last season we had a leak in the steps, which are about 10' from this area. No suspicious areas between the steps and this hole. Any comments or suggestions on how to deal with this? The liner seems to be in good shape and I really don't want to have to replace it.