You won't find it on your tube map, but there's an underground station beneath the streets of Clerkenwell. This is Mount Pleasant, one of eight stops on the Mail Rail tube line. The private route was used by the Post Office to transport letters and parcels across the capital from 1927 to 2003.
We recently took a tour of Mount Pleasant station, including a journey through the tunnels in a passenger car. The station and tracks will be opened as an attraction at the end of the decade by the British Postal Museum and Archive. A new museum will open in 2016.
We also recorded a second video, showing an eight-minute journey through the tunnels.