70 or so feet underneath London's streets lies the unused mail rail system, a hard to reach subterranean system of postal tracks, cloistered away from human eyes. That is, until a gang of intrepid urban explorers cracked a secret entrance.

After a precarious climb up a five-story drain pipe, the spelunkers at Silent UK plumbed the depths of this system, which has existed in various forms for over a century:

Originally designed using a pneumatic system in 1855, after years of testing, tunnel construction and usage its limitation began to show. The Post Office who were already unhappy with its high running costs, coupled with the fact the scheme only shaved four minutes from the delivery time by road decided in 1874 that they would no longer use the line, the Pneumatic Despatch Company being dissolved as the tunnels were closed [...] Even before the demise of the Pneumatic line, several plans had been put forward recommending a similar mail delivery system, most promoting usage of electrified lines [...]

You can see many more photos of these mostly inaccessible tunnels over at Silent UK. Hat tip to Spencer!