Pennwood, you've gotten some good advice. I agree with the idea of pumping out the hole to see if it refills. That will tell you something, either way. If it refills, you're most likely at a water table. If it doesn't refill, that means the surrounding soils are porous and your water is making its exodus laterally through the bottom.
Right now, it seems we are playing "ifs and buts"...however, I have a suggestion for you. If the pond bottom soils are, in fact, porous, you can line the pond bottom with clay as good as what was used in the dam. Odds are high that you have clay on your property, since that's where the clay came from to build your core for the dam.
If my idea is feasible, here's how I can see it work.
Using a scraper to move another two feet of soil off the entire pond bottom, use that dirt to fill a pit that you'll need to dig to supply your clay. Use the scraper to pull the clay from your property, bring it to the pond bottom and use a bulldozer to spread the clay in 4-6" lifts. Use a fully loaded scraper to compact the new clay in the pond bottom by driving back and forth over it.
When the clay is moved into the pond (be sure it has proper moisture) bottom and compacted, use the excavated dirt to fill the newest pit.
A natural clay liner is much less expensive and it's not hard to do, if you have the clay.
To help understand and project the numbers, one acre, one foot deep, is 1,667 cubic yards of dirt. To line the entire bottom of a 3/4 pond two feet thick will mean you'll need about 3,000 cubic yards of clay.
Anyway, stay with us, keep feeding some facts and you'll get some good answers.
Also, invite your wife to come with you to the Pond Boss V Conference and Expo, October 11-13, at Big Cedar Lodge on the shores of Table Rock Lake in Missouri. The place will be loaded with experts, professionals and vendors who pretty much cover the gamut of pond building and management. Plus, my wife, the Queen, will have some cool spouse activities. We'll win her over...and you'll come away refreshed and loaded with knowledge you didn't even know you needed.
