Bentonite is basically a type of clay that swells greatly when wet.

I've actually thought about lowering the pond level as you suggest (although I'd like to be able to do it simply by starting a siphon if possible). I'm not sure, however, how to go about patching the hole once everything is dry. How big of an area do I dig out? How do I plug the area? Do I merely use concrete?

Given that I've had problems with new holes forming that appear to lead to the same underground channel it seems like I'd do better if I could plug the channel instead of merely plugging the hole to it. I'm not sure if this is possible. I've thought that maybe if I got some pelletized bentonite maybe I could use the water that is funneling into the existing hole to carry the pelletized bentonite into the channel a ways before it would start to swell and plug the leak. The bentoninte powder that I've used in the past does a pretty good job of plugging the hole but if a new hole forms to take its place it sounds like I'll never get the pond permanently fixed this way. Of course, I may be wrong in all of this. Anybody out there have any ideas/hints?