The Construction News feature this week is fascinating (if you have a subscription you can read it by clicking here!)
Stretching across London are a series of mail tunnels - used by the Post Office to transport letters and packages across the capital. Built in the 1920s, they ran for 22 hours a day, covered 10.5 km and delivered over 4 million letters a day to different sites in London.
And, from 2017, 1 km of this network will be reopened as part of the new Postal Museum development. A ride within the museum will take visitors around the tunnels, allowing them to see first hand the amazing engineering feat that has been hidden under London for years. The video below gives you a feel for the scale and complexity of the project.