Ugh... So last summer (2015) we had an 18'x36' vinyl in ground pool installed with completion of the pool in August. All has been going fine until I noticed one of the long sided walls (36' side) seemed to have dirt behind the liner (between the steel wall and liner) which was not present before. In addition, my daughter discovered a sinkhole in the deep end of the pool where the angle of the drop meets the bottom of the pool. The bottom of the pool is made with vermiculite. The sinkhole is about 5-6" across and 2-3" deep. We are so disappointed. Not sure if this has anything to do with it, but about a week before we noticed all this, we had a large amount of rain that filled the pool to the brim. Could water be getting behind the liner? I've contacted the pool builder, but so far no response....