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 took a of Mount Pleasant station, including a journey through the tunnels in a passenger car. The station and tracks will be opened as an attraction in 2017 as the British Postal Museum and Archive.
We also recorded a second video, showing an eight-minute journey through the tunnels.