Thanks for all of the input guys.
Here's the latest. We have started to drain the pond and took a post hole digger and shovel to see how far we could dig before hitting clay (assuming there was some kind of liner).
Unfortunately we dug less than 6" and hit rock. Everywhere. For 15 years the pond was thriving but only in the last year has it leaked water so that now it's almost dry.
What do we do now?
Scrape it out, then call TJ (teehjaeh57) here on the forum for the polymer product that he sells to seal leaks. It will find the places where the water is seeping out and plug it. Similar products are used in the fracking industry to make water tanks for their drilling operations where water is brought in via tanker truck.
