kempo

Hello; We're getting ready to bring in a contractor to figure this leak out and I'd like to hear some ideas from you knowledgable folks, so that we only fix what's neccessary. Last spring when filling the pool, the water reached a certain level (almost full). Then all of the sudden the water level dropped very quickly and almost emptied. I tried this a few times and the water leaked out very quickly each time. The level that the water stopped leaking out was pretty much at the same spot each time. However there is no indication on any cracks and certainly not a hole big enough to leak as quickly as it does. The deep end is 9ft deep and the main drain has a relief valve(?) to allow ground water in to prevent cracking the gunite. Any ideas would be greatly appreciated.