Menwith Hill is an NSA complex in England that monitors electronic traffic across Western Europe, Russia and the Middle East. It's part of the infamous (in cypherpunk-y circles at least) ECHELON network.
The Menwith Hill site was loaned to the US Government in 1951. It opened for business in 1956 by the US Army and was operated by military intelligence until it was turned over to the NSA in 1966, who continue to run the site. Around 1,200 civilians currently work there.
Menwith Hill's cover story is that it's simply a Department of Defense communications station. I guess what they're not saying is that the "communication" they focus on is all incoming.
Menwith Hill is located at 52o 52'' N, 3o 3'' W.
External links:
http://www.fas.org/irp/facility/menwith.htm
http://www.menwithhill.com