
Few passengers careening between tube stations know there is an underground city beneath London.
In the 19th century, digging deep-level underwater tunnels was hazardous. Numerous attempts to cross the Thames underground had failed, with many lives lost. Today, the East London Line uses Brunel’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 was …