Hawthorn, Wiltshire (also known as Burlington, TURNSTILE or Site 3) is a 35 acre underground city / nuclear bunker that's about 30 metres below the Wiltshire countryside. It has a railway station, BBC studios, government headquarters, telephone exchange, pneumatic tube system, PA system, electricity generators, water purifiers, and sealable air ducts. The living facilities can provide accommodation for up to 4000 people. Hawthorn has since been decommissioned but was an important part of the emergency plans in case of nuclear strike during the Cold War.

I love all of these sorts of secret places, so it was with much interest I've found the BBC has a video 'drive around' of the complex along with panoramic tour photos.