The "Kingsway Tunnels" are currently up for sale in London. If you fancy an underground lair, and have around £5 million spare, then these could be for you.
Access to the mile-long system of horizontal and vertical shafts is through unmarked doors in the street on High Holborn, and the site is fully equipped with electricity, water supply and ventilation equipment - making it the perfect place to hole up and hide from 007, though aspiring Blofelds will have to provide their own white cat.
The Public Record Office used the tunnels for a while to store 400 tons of secret documents, before the complex was turned into a "trunk exchange" to connect long distance telephone calls in the days before the subscriber trunk dialling (STD) code.
It was built to hold 8,000 (!!) people during air raids, and in its days as an exchange housed around 80 workers, who enjoyed a canteen and recreation room - complete with snooker table.
More details here: http://www.24dash.com/news/communities/2008-10-15-For-sale-Secret-network-of-tunnels-100ft-under-London