The site is known as the Kingsway Tunnel (a deliberately misleading name, because Kingsway is hundreds of yards away) was originally dug in 1942 as a bomb shelter for 8,000 people during World War II. The tunnels were then taken over by military authorities as a communications and intelligence centre and after the war became a place to store historic documents.
They're now being put for sale by their current owner BT.
According to the Associated Press - "BT spokeswoman Gemma Thomas said that the company no longer needed the tunnel because the Internet was cutting down on the need for telephone exchanges. She said restrictions on the tunnels' use meant they could not be converted into a cool new concept hotel, an underground office or a subterranean home. BT suggested they might be suitable for government use or for a major corporation. Thomas refused to reveal was BT was hoping to get from the tunnels' sale."
BT are also remaining fairly cagey about how you find the tunnels. You won't find the entry door unless you know where to look.
Andy McSmith from The Independent was one of the journos lucky enough to get a tour of the secret site and said
"This would be a fabulous place to open a nightclub, but health and safety laws would never permit it. Likewise, a hotel, offices or living accommodation are out. Somebody should grab the chance to use the tunnels as a film location, because they are eerie, dated and full of places to hide away from the bad guys. Although much of the kit in use there in the post-war era has been removed to museums, I saw enough relics to make it a trip back in time. There were machines bearing names of manufacturers long since wiped out by mergers and recession, such as English Electric (Stafford), Metro-Vickers, Hackbridge and Hewittic Electric, and Ruston and Hornsby. The huge tubes forming the walls of the main tunnels bear the initials LPTB, meaning they were built by the old London Passenger Transport Board."
Now they are up for sale, BT are obviously trying to get some renewed interest in them again, but their secrecy was compelling for former reporters who were desperate to know what government secrets were going on down there.
"In the 1980s, the investigative journalist Duncan Campbell, then of The New Statesman, did not take no for an answer. John Tasker, the former manager of the Kingsway Tunnels, said: "He had an obsession about government secrecy. He phoned BT security one Christmas and told them 'if you go down, you will find a Christmas tree'. He had planted it. They still don't know how he got down there."
In the show Griff Rhys Jones hazards a guess as to how much the Tunnels might be going for (£5 million), but I'm sure with such limited commercial uses it'd only be in the price bracket of the mighty rich. Let's just hope that whoever buys it has an imaginative use for it & that more people get to see the eerie underground maze.