Menwith Hill is the largest electronic monitoring station in the world. It is run by the US National Security Agency (NSA), which monitors the world's communication for US intelligence. Menwith Hill employs 1,200 US civilians and servicemen to work around the clock inside "hardened" buildings intercepting and analysing communications mainly from Europe, Russia and the Middle East. Until a few years ago, the existence of the NSA was a secret and its charter and any mention of its duties are still classified. But, it does have a Web site in which it describes itself as being responsible for the signals intelligence and communications security activities of the US government.

All telecommunications traffic to and from Europe and passing through Britain is intercepted at the base, including private telephone calls, faxes, emails and other communications. Much of the information is collected, processed and relayed back to the United States automatically. A great deal of this information comes from spy satellites and the base has a number of large white golfballs or "radomes" containing satellite receiving dishes.

In its first decade the base sucked data from cables and microwave links running through a nearby Post Office tower, but the communications revolutions of the 1970s and 1980s gave the base a capability that even its architects could scarcely have been able to imagine. With the creation of Intelsat and digital telecommunications, Menwith and other stations developed the capability to eavesdrop on an extensive scale on fax, telex and voice messages. Then, with the development of the internet, electronic mail and electronic commerce, the listening posts were able to increase their monitoring capability to eavesdrop on an unprecedented spectrum of personal and business communications.

And the base keeps growing. Since June 1996, Menwith Hill has lodged two dozen development applications with the local council for eight additional radomes and a string of unidentified structures (the NSA describes its proposed buildings as "P6" and "Hazardous storage facilities", without naming the hazards).