What a bummer. Makes me wonder if there is a sand vein or similar just under the pond basin, and water is making its way in through multiple or a general location. I hope you find the problem and get it fixed! I too would enlarge the pond, but in so doing create a fairly deep keyway under where the dam will go, and pack it solid. It will give you a chance to see what is down there and hopefully find and block any sneak paths at the dam.
