UAC constructed a bolt-hole on the dark side of the moon which was terraformed for the purpose. It was known as the Waterwheel because of its shape. When you completed the cleanup at Deimos you were warped to the Waterwheel because the infestation had spread even there. You find yourself transported to the centre of the Waterwheel and need to find your way to the exit warp-point cleaning up as you go.
Sadly, the hell spawn has already carved holes into the roof and the upper levels have been flooded. Fortunately, door blocks have prevented the flooding from spreading to the control rooms that make up the wheel(s) and the lower levels where the bugs from hell have constructed their dead-rooms.
Not a lot to like here. A handful of OK maps buried in a sea of absolute dreck. The last third of the wad was particularly awful - just skipped through that in unfounded optimism there would be at least one map that was interesting/playable. A last resort mapset when you have nothing else to play.
This needs a warning label - it has one of the most annoying "enhancements" ever to stain Doom. A countdown timer. I just abandoned the level when I noticed it. Also first hall you enter has a dozen Hell Knights and god knows how many Imps, while you have a single shotgun and a dozen shells. This kind of crap makes me want to punch my screen. So you've been warned.
I always felt like the vanilla Doom II experience was lacking a certain little something. Stick it in your GZDoom's "skins" directory, you will never regret it. True magnificence is something which chases you around.