This was a joint CIA-MI6-KGB operation to tap the phones in the East Berlin main telephone exchange. It required digging a 450-meter-long tunnel from a building the CIA and MI6 erected in West Berlin to the East German dial central office, tapping the lines, and building a wall (complete with a sign in German and Russian that said "entrance is forbidden by the commanding general"
between the East Germans' equipment and the Western equipment monitoring the lines. The KGB learned about the tunnel before they started digging it from British double agent George Blake, but they knew breaking up the operation while Blake was still stationed in Berlin would have led to his arrest and they decided Blake was more important than anything that could have been on those lines...so, they waited until Blake had been reassigned before they "discovered" it.
By comparison, all they had to do to pull this off was to violate the sovereignty of about nine countries by flying bombers over them.
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