Thanks for all the help everyone!!! After poking around the site and getting all this great information, I drained the pond today and notcied where the ground level met the soil I added on top of it for the dam, I notcied that is where the leak stopped while draining. I also noticed there were roots from small trees on this line. I have decided to do the core from the inside once it has dried out a bit for the enitre dam. How far should I dig under the previous dam before begining my job? Again thanks for all the help!