Tunnelling machine Modern tunnel boring machine ‘Matilda’. © Erikt9.

By Professor Robert Mair FREng FRS
Professor of Engineering, Cambridge University

What are London’s secret and unknown tunnels?

There are all sorts of tunnels under London as well as the Tube: there’s the abandoned stations and air raid shelters which get used in TV dramas and films, the complex under Whitehall which includes the Cabinet War Rooms, the famous series of tunnels under Harrods, the post office railway (also known as Mail Rail - closed in 2003) and that’s besides the 82 mile sewer system created by the great Victorian engineer Joseph Bazalgette.

Who was the first to tunnel under the Thames?

The first tunnel under any navigable river was Sir Marc Isambard Brunel’s 396m long Thames Tunnel- construction was supervised by Sir Marc’s son Isambard Kingdom Brunel. It was a catastrophic project, taking 18 years to build due to collapse of the ground into the tunnel - the resultant flooding, fire and leaks of sulphur and methane into the excavation resulted in the death of 6 men. It is still used to this day as part of the East London Line.

How has the engineering behind tunnelling developed?

New technologies have greatly reduced problems and increased efficiency - whilst it took the Brunels 18 years to complete the Thames Tunnel, modern tunnel boring machines can advance at a rate of up to 50m per day in total safety.

How many miles of London Underground tunnels are there?

London Underground has 400km of track, of which 182km are in tunnels. Servicing this track are 270 stations, each of which has its own network of tunnels and shafts. There are so many tunnels beneath London that new projects such as Crossrail will have to construct tunnels through the remaining underground space.