Unknown at the time to the CIA and MI-6, the KGB - the Soviet Union's premier intelligence agency - had been aware of the Berlin Tunnel project from its start. George Blake, a KGB mole inside MI-6, had apprised the Soviets about the secret operation during its planning stages. But to protect Blake, the KGB allowed the operation to continue until April 1956 when they "accidentally discovered" the tunnel while supposedly repairing faulty underground cables - without putting Blake at risk.
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