There is a veritable city's worth of tunnels underneath London. Many of these tunnels are secret tunnels, decommissioned military tunnels, decommissioned subway tunnels, postal tunnels, and much more. No one knows the full extent of the tunnels under London.

LONDON — In 1913, the British Post Office realized that London’s congested traffic was slowing down mail delivery and something needed to be done about it. That year, Parliament approved building the Post Office Railway, as it was officially known, and ...

Snail mail” was anything but slow in London in the 1920s. In fact, a letter would rocket beneath London’s slow-moving streets as fast as 64km/h, courtesy of a little-known subterranean narrow-gauge rail line that ran for 35km between the capital’s letter-sorting offices.