LONDON: Once home to British spies and a secret long-distance telephone exchange, a little-known network of underground tunnels is located behind a blue door in a narrow London hallway.

Thirty metres (100 feet) below the UK capital’s bustling streets, all that can be heard in the tunnels built to withstand a nuclear attack is the rumble of the London Underground’s Circle Line.

A sprawling network of secret underground tunnels is being transformed into a major new London tourist attraction. Hidden 40m below the streets of the capital, the Kingsway Exchange Tunnels were ...