mental_floss presents brief histories of five spy tunnels. Tunnels were a major spy tool through World War II, but most of these underground projects ended up compromised by double agents, or being abandoned all together due to the difficulty and price involved in their construction.
The few effective tunnels made it clear why intelligence agencies such as MI6 and the CIA attempted to build them in the first place:
“In 1944, the tunnel shelter at Chancery Lane was allocated for the Inter Services Research Bureau, an arm of MI6. The ISRB initially had intentions of helping the German resistance. Soon, however, it was a hive of 10,000 covert operatives. In 1945, MI6 vanished from the facility, leaving no evidence of its once thriving presence. Questions still remain as to what the massive spy apparatus did beneath the streets of London that year, anyway.”