In the 1950s, the CIA and Britain's MI6 dug a secret tunnel under Berlin to tap Soviet phone lines — not knowing a mole had already betrayed it.

Operation Gold ran a listening tunnel beneath the border to intercept Soviet and East German military communications. Unknown to the diggers, a Soviet spy inside British intelligence had tipped off Moscow before it was even finished — yet the Soviets let it run for a time to protect their source, making the haul of intercepts genuinely useful anyway.