A Soviet soldier inside the tunnel in April 1956.
As the CIA opens its classified archives and even puts formerly secret information online, the resulting treasure trove has given historians and intelligence experts lots to debate.
Were some of the agency's post-World War II plans brilliant or absurd?
Sometimes they appear to be both.
The CIA recently opened its files about a huge secret tunnel that U.S. and British intelligence planned and built under the center of Soviet-controlled East Berlin.
At 1,476 feet, it was no small project: 3,100 tons of soil were removed, and 125 tons of steel liner plate and 1,000 cubic yards of grout were brought in. The objective was to tap into Soviet and East German phone lines in order to eavesdrop on secret communications. It took seven months to build, and it operated for 11 months, before the Soviets shut it down.
But the Soviets actually knew about it from its earliest planning stages. While teams of soldiers were underground digging like moles, British intelligence was plagued by a different kind of mole — a spy — who told the Soviets all about the tunnel. Still, the Soviets let the construction and operation of the tunnel proceed unhindered.
'An Unanticipated, Messy Problem'
The planning and construction of the tunnel was endlessly complicated. Great pains were taken to store the displaced soil so as not to arouse suspicion. One of the tunnel's planners, who worked on New York's Brooklyn Battery Tunnel, wrote on the CIA's Web site that workers encountered "an unanticipated, messy problem" when they tunneled into their own sanitation system.
At another point, engineers realized that the tunnel was becoming warmer than they had calculated, a situation that could have created an emergency. Engineers were petrified that on a cold morning, a frost-free area on the ground could call attention to the tunnel. They quickly increased air conditioning levels.
In the end, the Soviets did decide to shut down the tunnel. But why they waited so long is one of the great unanswered mysteries of Cold War espionage, says Tim Weiner, Pulitzer Prize winning author of Legacy of Ashes: The History of the CIA.
The Soviets didn't want to blow the cover of their mole, George Blake, but that doesn't fully explain why they waited so long. Nor did they use the tunnel to feed disinformation. In fact, the tunnel yielded a considerable amount of useful intelligence to the U.S. and Britain. "They kind of got a blueprint of the correlation of forces on the other side of the Iron Curtain, and they learned who talked to whom, who reported to whom," says Weiner. "It was a wiring diagram, if you will, of how the other side worked."
Blake continued to work for the Soviets throughout the 1950s. He was eventually discovered and imprisoned, but managed a dramatic escape. He fled to the Soviet Union, where he later died in Moscow.
Coup or Folly?
Intelligence experts continue to debate the tunnel. Some look at it as a success, both from an engineering standpoint and from the 11 months of information it yielded; others see it as an enormous misguided mistake.
"I think you can read it two ways," says Weiner. "Was it really, really neat that they could dig 500 yards into occupied territory and tap into telephone cable? Yes. Were they thrilled at the time at their accomplishment? Absolutely."
"But," he asks, "Did it make any difference in the end? Not really. This is the problem in espionage. You can score tactical victories in the short run, but strategically it's very hard."
"There are," Weiner says, "very few great triumphs in espionage."