FishinRod.

Thanks for the great advice.

I've had a few people come out and look and the best theory is that there's a crack in the bottom of the pond that facilitated the leaking. Our area is full of limestone rock and when wet creates karst which tends to be porous. I don't know much about rock, but this explanation seems the most plausible.

I've decided to just go ahead and drain it so we can see what's under there. As is, the "pond" is just a muddy mess and makes for dog play that creates work with all the cleaning up. Until I can get it to where dogs can jump in and swim, it's just a sinkhole.

Appreciate your help in thinking it through...