The entire bottom of the pool had to be reworked, as you can see from the picture below the previous owner had neglected the pool for so long that there was a tree growing out of it... The bottom of a pool is a special type of concrete that "breathes" so that any water that comes from below, can go back down when the water table goes down. If you didn't do this, when the water table rose the pool would basically float up out of the ground
The entire process took a couple of weeks, and we had to use extra thick foam on the walls to protect the liner from the slightly rusted side walls.
In the end though, it was totally worth it, we were able to move the pump from the back of the house to the back of the lot, and Gavyn lost 12 lbs in a week and a half of swimming...:)