Few passengers careering between tube stations know there is an underground city beneath London.
In the 19th century, digging under-water deep-level tunnels was hazardous. Numerous attempts to cross the Thames underground had failed, with many lives lost. Today, the East London Line uses the Brunell’s Thames Tunnel, being the first successful under-Thames crossing.
When steam traction was exchanged for electricity, deeper tunnels could be dug using compressed air and a large circular drilling shield, which …