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.
Ben Morris for his superb editor, DCK, Jens Hykkelbjerg for the excellent Reject Map Builder RMB version 2.1, the DEU Team, and, of course, the wizards of Id.
New level built from scratch
Doom Construction Kit 3.2 by Ben Morris WinDEU consistency checker RMB version 2.1 reject map builder