Fibreglass Pools, Inspection Pipe.
Some (Fibreglass mostly) pools may have an inspection pipe(s) near the pool. Usually about a metre from the pool edge. These would have been installed at the time the pool was placed in the ground as a way to check ground water levels near the pool. (one can assume ground water was an issue at installation time) If you are thinking of emptying a pool have look and see if you can see any nearby. They are usually closed off with something like a shower grate to stop too much rubbish filling them up. Most likely it will have become sealed through neglect and you will have trouble opening it. However you may be able to check the ground water level by inserting a thin dowel or stiff wire down and then measuring the depth. As a guide if the water level shows as being LESS than about 1.5 to 1. 8 M below ground level be careful when emptying the pool. It may cause movement or bulges. (see picture)