David Hay, a BT historian, said legend had it that the government wanted to keep the location of the tunnels so secret that it hired foreign workers with no knowledge of the London streets to build them. BT staff members are still under strict orders not to reveal the exact location of the system, though incomplete maps have surfaced on the Internet.

“We just don’t know what the future owner will want to use it for, so we can’t disclose more information,” David Hembra, one of the maintenance workers who now visits the tunnels several times a week to check for gas leaks and other problems, said.

When Mr. Hembra started to work in the tunnels 10 years ago, their pivotal years were behind them, and little remained from the turbulent days of World War II. The offices were removed after the war ended, when new tenants moved in. Britain’s public records office needed the space to store more than 400 tons of documents.

But it was not long before the documents had to be moved again to make room for a secure international telephone center that the government deemed necessary as relations between Washington and Moscow grew tense. During the cold war, the British government instructed its telephone department, which later became BT, to set up a secret communications system based on the latest technology that would be able to survive a nuclear attack.

It was the beginning of the busiest period for the tunnels, with almost 200 workers spending their days and nights underground to route up to two million calls a week across the 6,600 phone lines. In 1963, the hot line established between Moscow and Washington after the Cuban missile crisis ran through the London tunnels.

The buzzing complex soon became known as “underground town,” with its own recreation room complete with dartboards and billiard tables, a movie theater and two dining halls. Workers often spent the night in sleeping rooms.

By the early 1980s, technology had advanced so much that the tunnels’ telephone center became obsolete, and BT’s technicians moved back above ground.