I have been working on a small backyard pond for some time now. It's about 4 feet at its deepest. I am not completely finished with it yet, and now I'm noticing something. There is clay at the deepest part. About a foot of rain has collected. I bucketed the water out TWICE...and both times I returned the next day to find all the water had returned...without any rain. Any ideas? I do not believe it is spring fed because it was dry when I was digging. Possibly ground water trapped above the clay? Anyone else ever have a similar experience?