Soviets Watched a Fake Warehouse While CIA Dug 1,500-Foot Secret Tunnel

By Harlan Pryce · August 9, 2026

East Berlin border guards stood atop a fake warehouse while agents hid 3,000 tons of sand to tap Soviet landlines, though records end abruptly before mentioning an individual named Blake.

The primary problem with digging a secret tunnel is not the digging itself, but the dirt. When the CIA and the British decided to intercept Soviet communications in East Berlin between 1952 and 1956, they faced a logistical nightmare: approximately 3,000 tons of sand. In a city where the border was "heavily and constantly patrolled," you cannot simply dump three thousand tons of spoil into the street without inviting the Soviets to dinner.

Collaborative Paper No. 150, prepared in 1967 and released via the CIA's electronic reading room, details the solution. The project personnel, according to the text, spent weeks discarding ingenious ideas until someone suggested facetiously that they "dig a hole and put the dirt in it." This became the plan. They built a two-story warehouse, half above ground and half below, under the guise of a Quartermaster Corps experiment in low-cost storage. The basement was dug "under the eyes of the local border guards," providing a convenient hole for the 3,000 tons of sand.

"Dig a hole and put the dirt in it"

The warehouse cover was a pragmatic piece of tradecraft. By presenting the opposition with a reason for the site's existence—a mundane military storage facility—the Agency argued the site became a less prominent target than a "mysterious something." The irony was not lost on the men on the ground; the paper notes with some amusement that Soviets and East Germans could be seen standing atop the tunnel with binoculars, focusing on the roof of the very installation that hid the operation.

However, the logistics were not all so clean. The paper admits that the placement of the onsite cesspool was "quite unfortunate," as it was situated only a few feet from the tunnel site. This ensured that working conditions in that sector were, in the writer's words, "highly unpleasant."

Vienna and the Vertical Shaft

The Americans did not invent the concept of the signal tunnel; they borrowed it from the British, who had already spent years tapping Soviet cables in Vienna. This shared experience created a division of labor. The U.S. provided the site, the warehouse, and the 1,500-foot tunnel. The British, leveraging their specialized expertise, drove the vertical shaft from the tunnel's end up to the target cables and handled the voice recording processing at a jointly manned center in London.

The engineering required a level of precision that left little room for error. To prevent the sandy Berlin soil from caving in, the team used a steel liner and a shield with horizontal "blinds." To ensure the ground didn't settle—which would have alerted the Soviets to a hollow space beneath their feet—they used high-pressure grouting to fill the void between the liner and the soil. The result was a structure capable of supporting the weight of heavy trucks on the highway above, though the crews below still felt the "weird sensation" of an iron-shod horse trotting across their ceiling.

"VRE CBS"

The impetus for the tunnel was a gap in intelligence coverage and the emergence of a Soviet voice secrecy device known as "VRE CBS." While the term normally meant "high frequency," the Soviets used it to describe a special speech scrambling device for high-level communications. The Agency needed raw access to the landlines to crack this system.

The technical success was proven early, though not glamorously. The first material recovered from the target signal was a recording of a student teletypist practicing on "home keys." It was an intelligence vacuum, but a technical victory: it proved the tap worked.

A Problem of Native Fluency

Despite the engineering triumphs, the project hit a wall when it came to the human element. Processing the intercepted Russian voice traffic required linguists with "near native fluency." The writer of the paper notes the axiomatic truth of the field: native fluency is usually only available in natives. This created an immediate conflict with the "strictest possible security measures" the project demanded.

The Agency was forced to balance the need for linguistic precision against the inherent risk of employing native speakers in a Top Secret operation. It is a tension any case officer recognizes—the more useful the asset, the more dangerous the source.

The paper describes the bilateral cooperation between the U.S. and U.K. as generally smooth, with "few, if any, problems." However, the writer makes one caveat, noting a "notable exception" regarding an individual named Blake on page 23. The released text of the paper ends before that page is reached.