Holy drain hole! that's impressive. Might even be a sinkhole? looks like it may go right down into a cave. I think I would try to pump all the water out to see if there are any other drains.

Then it sure wouldn't hurt to try packing it/them with a $300 worth of bentonite 1,000 lbs? Need to get the first couple bagfuls way down in there before any clogs temporarily plug it. Then probably better to dig out a 10 foot circle(?) with a trackhoe, lay in a impervious fabric layer to prevent soil going down if a suck-hole develops and cover it with more bentonite on it, then pack with clay. Then you haven't spent much $$ before the big cost of packing the entire pond bottom with clay.

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