
It was one of the most audacious projects attempted by the West during the Cold War: a vast underground tunnel beneath the divided German city of Berlin that could be used to spy on the occupying Soviets.

The 1950s project might had seemed absurd, but for the US and Britain it was essential - the Soviets had switched from radio broadcasts to telephone calls, which required a physical connection to overhear.

So began an immense two-year project between the CIA and Britain's MI6 that saw agents literally being sent underground - and was foiled by a dastardly double-agent, according to a CIA report.

Going underground: This is one tiny section of the near-1,476-foot-long tunnel built by US and UK agents into Soviet-controlled Berlin during the Cold War. The tunnel took from 1951-1954 to plan, and a further two years to actually dig

Tapped in: This end of the tunnel held the amplifiers and other equipment needed to tap into the Soviets' lines, as they had stopped using radios years before. The steel-walled tunnel could withstand a 60-ton tank driving overhear

Can we dig it? The tunnel began in a fake warehouse in Altglienicke, a US-controlled but sparsely populated area of Berlin. The equipment was rubber-coated to avoid being heard by the people above

Codenamed Operation Gold by the CIA and Operation Stopwatch by the British, the joint mission spun out of Britain's Operation Silver, which had seen MI6 tapping Soviet lines in Vienna.

But the Berlin job required some serious planning to pull off - and some serious hardware too.

In 1951 a CIA engineer, code-named 'G', was contacted by top CIA brass about a possible new project.

'The only question they asked was whether a tunnel could be dug in secret,' G later wrote in a now-declassified document was consulted on the tunnel in 1951.

'My reply was that one could dig a tunnel anywhere, but to build one in secret would depend on its size, take more time, and cost more money.'

G's previous experience was limited to just 'several' visits to the Brooklyn-Battery Tunnel that was constructed under New York's Upper Bay in 1948.

Nevertheless, G was put on the job, and was immediately posed with a near-impossible question: how to remove 3,100 tons of soil - enough to fill 20 American living rooms - from beneath Berlin without anyone noticing.

Worse, the soil could not be moved from the dig site, for fear of altering authorities.

The solution, G realized, was to move the soil to a nearby building - ideally a warehouse - with a basement that could be slowly filled with dirt.

There were other things to consider, too, like lining the tunnel with steel so it could bear the weight of the 60-ton Soviet tanks rumbling through the streets above.

Secret flights over the territory provided the architects with an idea of where to dig and specialist machinery, including conveyor belts, was created and coated with rubber to minimize clanking and other noise.

It was shipped in using unguarded passenger trains, so as not to attract attention.

The team built a 'warehouse' on the American Sector southwest of Berlin, known as Altglienecke. A tunnel would then be dug from the foundations through to the Soviet-occupied area, where three telecoms lines would be tapped.

Finally, after three years of preparation, the team were ready. It was a perfect plan, provided the Soviets didn't find out.

But unbeknownst to MI6 and the CIA, they already had.

Uncovered: The Soviets had been tipped off to the tunnel's existence by a double agent long before construction even began. Here, Soviet officers show it off to the press. The CIA argues that the infamy they gained from this was a boon to them

The dig began in 1954, with the team slowly but near-unstoppably (a brief encounter with a field used to drain sewage caused an understandable delay) gnawing their way through the ground.

Behind them, cables trailed off to the banks of recording machines on the ground floor of the warehouse.

Every so often high-pressure grout would have to be pumped into the two inches between the spoil and the steel plates, to stop the ground above sinking noticeably.

And as winter arrived, another problem raised its head: The heat from the tunnels left a clear line of warm, wet ground in the winter frost on the roads above.

Emergency air conditioning was fitted to keep the tunnel suitably frosty, and suspicion at bay.

It took a year to reach the cables - and then the really painful work began.

Listen here: This is a diagram showing the 'tap chamber' at the end of the tunnel. British sappers had to dig vertically using a metal 'shield' fitted with blinds. They would open the blinds to allow just one inch of soil through at a time

Divide and rule: This is the division between East and West Berlin, before the Wall was built. The provisional barrier can be seen in the middle

British sappers had to dig upwards to reach the cables - a painstaking process that saw them placing a 'window blind' shield facing upwards.

This would hold the soil above in place until one of the several blinds was opened.

The sappers would then remove just one inch of soil at a time, a process that, G said, 'required extreme patience and skill.'

Finally in May 1955, they were able to tap the first cable, with others following. British specialists transferred the voices from the cables to tapes, which were then sent to London and Washington for analysis. The final tunnel was 1,476 feet long.

For a year, the team were able to produce 50,000 reels of tape, 443,000 fully transcribed conversations, 40,000 hours of telephone conversations, 6,000,000 hours of teletype traffic and a total of 1,750 intelligence reports.

But what they didn't know was that the Soviets were on to them the whole time.

A mole within MI6 - George Blake, a secretly committed Marxist-Leninist - had tipped off the Soviets before the first ounce of dirt had been moved.

Double agent: George Blake (right, pictured with his mother in 1967) was a Marxist-Leninist and double agent. He told the Soviets before construction began, but they allowed a year of wiretapping from 1955-56 to let him keep his cover

The Soviets then turned a blind eye to the operation to avoid blowing Blake's cover.

But on April 21, 1956, after Blake had received a promotion that would put him in the clear, Soviet and East German soldiers broke into the eastern end of the tunnel.

The espionage was called a 'breach of the norms of international law' and 'a gangster act' by the Soviets, but the CIA says that the ingenuity of the operation, which was acclaimed around the world, did them more good than harm.

Blake continued to work as a mole until 1961, when defectors named him as a double agent.

He was taken in and made a full confession, eventually being given 42 years in prison - then the longest sentence ever handed out by a British court - but he escaped in 1966 and fled to Russia.

He lives in Russia to this day, on a pension provided by the country, and is now 94.

In 2007, on his 85th birthday, he was given the Order of Friendship by Vladimir Putin