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.
I was involved in the design and play-test of each of these levels. Please know these were created from *scratch* and we spent our time making sure the levels were as playable as possible. We were not terribly focused on texturing, other than to give players positional awareness in different parts of each arena.
Map 6 -- Offices of Death -- Was intentionally designed to be as accurate a representation of the office we worked in as possible. Admittedly, playability was taken as a second priority. We did manage to create working "elevators" and "stairs" to give the illusion you were going up and down floors. I was pretty happy with how we did.
Considering it was 1996, and no way to simply "Google" different ideas. Your best bet at the time was to jump onto the Usenet and hope you could find a decent board with active members. Other than that, it was pretty much all self-taught.
I'm glad you enjoyed the other levels. Yes, they are old-school, big arena, style. We spent a LOT of time trying to make sure they were as balanced as possible, with escape routes, and required risks for rewards.