In the back underneath the overhanging runway. If you see the wall with sea lanterns on top, you're in the right area. Staircase to the right and another entrance to the left. Or through the iron doors on the upper runway in the back, into the large tower.
I saw this on the live map when I went out to raid the wastelands there. I turned the live map on and saw it instantly before I could click on my name. I was quite a ways from spawn, so I switched to 3D mode to check it out. Very impressive from the map, I'll have to check it out in person sometime. I just hope I don't accidentally drop a fish and end up married to it .
