A question I would have on the pond leaking, draining down, how long of a period of time did it take for the pond to drain down to that point? the answer to that question would give a better idea of the chances of sealing that off permanently, if it was a slow leak that took weeks for it to drain down to that point it should be easier to fix, however if it went down over night you have a lot bigger hole below the surface that has eroded away and will be a lot harder to repair, I had a fellow here local that had a ten yr old pond drain out over night, assume drained into some underground cave, Im not sure if he ever did get that fixed permanently, I know they pumped a lot of grout into the hole trying to close it off. Good luck!
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