I've seen this build on many servers for some reason
I built most of it during the v9 beta. Rotondo built a small cave in the lake and there's a few other things people built that I don't remember. It was in the v9 release party server for a while and that's probably where people got it from. I had built a breakout game somewhere in it but this looks like an older version. The thing labeled waterfall in the OP is actually a dam and those falls are supposed to be the spillways.
It's there for everyone to use and for everyone to play with, you're welcome to do whatever you want with it. Sell it, put your name all over it, we don't really care.
That door and the control room are new though.