A mystery

A weird thing happened the night before last. We have a small goldfish pond, 2 meters square with an average depth of about 0.5 meters - i.e. it holds about 2 cubic meters of water. When I went to feed the fish in the morning, the water level was about 8 inches below its normal level. At first, I though there must be a leak; but the level was not falling, so I discarded that idea.

Having thought about it, I think someone must have come in to our lot and taken a lot of water. Were they wanting the fish? We have so many, hundreds of small ones, I could not tell if any were missing. The two big ones we have were still there.
Problems:
1) the amount of water taken was about 2 x 2 x 8 / 39 cubic meters - about 0.82 cubic meters. This amount of water weighs 820 Kg. There must have been a lot of hard work removing 820 Kg of water in the middle of the night. They could have siphoned it out with less work, but they would not get many fish that way, if any.
2) What did they do with the water? The ground was wet with rain, we could not see any signs of excessive water nearby.
3) Why did our three dogs raise no alarm? They are normally good watch dogs and they were out there in the yard, free to roam about. My wife went to bed after 01:00 and I woke and got up at 03:15, so there were only two hours when there was no one awake that night to hear a disturbance.
4) Who would want so much water and what for? Anyone wanting water had only to draw it from the well a few yards away and easily accessible, there is no shortage of water here at this time. If they wanted the fish, what for?

My wife thinks it was not any human that did it. OK, what then?

The pond is now full again, we have had rain since then and a pipe from the gutter provides a lot of water during a shower.