It was built during the cold war to house miltary and civil servents, should a nuclear war start and has nothing to do with aliens. If you ever watched those shows done by Mark Thomas, in the episode revealing the secret map of Britain he makes a reference to it.
Many of you will already know of this, so I'm sorry if I'm repeating what you already know. Basically when you're driving along a motorway, on a road bypass, down some back road, etc., you often see side roads which seem to lead off to nowhere. These are often marked private or unmarked with a barrier gate blocking them. Naturally some lead to private houses, others lead to storage areas used by the local council to store vechicles, materials, etc. But a number lead to areas used by the military and aren't on maps. The show also reveals the purpose behind strange looking signs which you may have noticed by the side of the road. Watch it because you'll learn a few things.http://video.google.com/videoplay?docid=2130977739763182534
As for the Men in Black: Have you realised that if the M.I.B. started wearing Pork Pie hats they'd look like the Blues Brothers?