Ulrich Pfeifer lay in the cramped, soaking wet tunnel he had helped dig 145 metres under the Berlin Wall, and prayed that his measurements were correct.
After three months of backbreaking work hacking away at the tough clay soil, that final moment, as his friend gingerly pierced the floor of a cellar on the eastern side, was the most dangerous and tense. Would they come out in the right building? Would they find themselves staring into the gun barrels of East German agents and be shot, or dragged out to serve years in jail? Mr Pfeifer, 26 at the time and a trained construction engineer, had got his surveying right. A total of 29 men, women and children, most of them friends or relatives of the 40 diggers, crawled to freedom from the cellar of the apartment building at Schönholzer Strasse 7 in East Berlin on September 14 and 15, 1962.
"Tunnel 29," as it came to be known, was one of the most famous and successful of the 70 tunnels dug under the Wall. "It was an incredible feeling of joy after so many months of work. I felt such satisfaction at having done something against this system in the East. I hated it," Mr Pfeifer, now 73, said in an interview in his flat in West Berlin. "We were so motivated and felt so much emotion that we had to do something to get rid of this feeling of powerlessness," he added.
Risking his life and his liberty to help easterners get out was a form of revenge against a dictatorship that had jailed his girlfriend in 1961 for trying to reach him in the West. Germany's celebrations marking the 20th anniversary of the fall of the Wall on November 9, 1989, have re-awakened memories not only of the historic night when communism in Europe was toppled, but also of the hundreds of escape attempts, many of which ended in tragedy.
At least 136 people died trying to cross the Wall, according to the Berlin Wall memorial centre. Other estimates are far higher. Mr Pfeifer worked on four tunnels. Three of them were betrayed, in line with the statistic cited in the book The Escape Tunnels of Berlin that only one in every four tunnels built was successful. And although a total of 250 East Germans managed to flee through tunnels during the Cold War, far more were caught in the process and subjected to long prison terms.
In September 1961, just three weeks after communist East Germany had shocked the world by erecting the Wall to stop its citizens deserting the repressive state, Mr Pfeifer had managed to escape to West Berlin with five others by crawling through the city's sewage system. "The people I went with said six was the maximum because they had to move fast. So I told my girlfriend that I would go ahead and immediately make sure that she could come with the next group a few days later," he said.
In those early days of the Wall, the Stasi, East Germany's secret police, was still so busy sealing off the city with concrete and barbed wire that it had not plugged all the escape routes underground. But it caught up fast, and Mr Pfeifer's girlfriend was arrested after trying to flee through the sewers 10 days later. "She got seven years' jail. It was an insane sentence. After 14 days of interrogation she testified that she worked for the French secret service, which was of course total nonsense," Mr Pfeifer said.
"The Stasi worked on her so ruthlessly, threatening that she'd be kept in jail for life, that she ended up signing whatever they put in front of her. The sentence later got reduced to three years. But I didn't see her again until 1988." Mr Pfeifer's former girlfriend married another man and had a family in the East where they still live. Mr Pfeifer remains friends with her and they meet often. "Tunnel 29" was only used for two days because the diggers, most of them students, feared it would be quickly discovered. It also was rapidly filling with water from a burst pipe. The escape made international headlines and the Stasi did not find the tunnel until 11 days later when the ground caved in on some of it.
Flushed with success, Mr Pfeifer joined an even more ambitious venture to get 50 people out through a longer tunnel that was to run from the same spot - the cellar of a bomb-damaged factory building in West Berlin, just across the street from the Wall. To prevent flooding, they built it deeper, at eight metres, and dug at a slight incline so that any water would run off. Mr Pfeifer helped Joachim Neumann, a friend of his, with the tunnel surveying, and took charge of security, recruiting couriers to go over to East Berlin and make contact with friends who wanted to flee.
"We knew we had to be even more secretive because the Stasi was on alert. This time the diggers lived in the cellar of the factory so they wouldn't attract any attention. "We found seven or eight couriers who all had to be foreigners or have West German passports because they were the only ones who could get visas to enter East Berlin. West Berliners weren't allowed in. The couriers needed a bit of courage because there was always a risk."
Mr Pfeifer's security arrangements for meetings between couriers and prospective refugees in East Berlin sound like a Cold War spy thriller. "We arranged code words and secret signs such as holding a newspaper in a particular way," he said with a smile. "All it took was a bit of imagination." They placed an encrypted sign on a car to lead refugees to the tunnel. It read "Brunhilde Nenkowski Cosmetic Salon". The first syllables of the first two words made the word Brunnen, the name of the street where they should enter the cellar of apartment building, Number 45.
On February 18, 1963, the 150-metre tunnel had reached the cellar floor of Brunnenstrasse 45 and Hasso Herschel, the lead digger, gingerly pushed up a paving stone, just enough to make a small hole through which to peer around the cellar. Everything was quiet. All the other diggers retreated back down the tunnel so he would have room to flee back West if something went wrong. There was a faint light coming from a small window and as his eyes got accustomed to the gloom, Herschel could make out a pillar. Then he heard a strange rustling.
"There was absolute silence for minutes. Suddenly he had the impression that a shadow was moving very slowly behind the pillar," wrote Maria Nooke, a historian, in a book about the tunnel. After further endless minutes, he could make out the outlines of boots. Realising that Stasi agents must be in the cellar, he quietly crept back West. The tunnel had been betrayed. "We had worked so hard to keep this secret that we were optimistic it would succeed," Mr Pfeifer said. "But you could never rule out that a friend worked for the Stasi. You're helpless against such things."
Several couriers were caught in East Berlin and arrested, along with many refugees they had contacted. At least 25 people were sentenced to up to four years on trumped-up charges that they had risked the outbreak of a third world war. It did not emerge until long after the fall of the Wall, when the Stasi files became accessible, that the tunnel was betrayed by an agent who had infiltrated a circle of friends in East Berlin trying to escape.
"The Stasi used all the means at their disposal to prevent the tunnels, and eventually employed seismographic and acoustic equipment to try and detect them," Ms Nooke said. They even conducted geological surveys to locate areas of particular risk. "Two tunnel builders were shot dead in separate incidents, and one Stasi officer was shot by a builder during an escape." Western intelligence services knew about some of the ventures, but did not interfere, partly because they saw them as endangering tense relations East and West.
Mr Pfeifer's closest shave came in August 1962, when he briefly helped to complete another tunnel. "We came out in a house that didn't have a cellar and had to smash through the wooden floor of a ground-floor apartment. But it had already been betrayed. We looked out of the window and could see the police trying to get into the house. We got away at the last minute." He worked on one more tunnel in 1971, but the Stasi got a tip-off and detected it with ultrasound before it was completed. The last known tunnel was dug in 1982, but it was betrayed and only reached a length of 5.5 metres.
Mr Pfeifer, who married and led a prosperous life in West Berlin as a construction consultant, remains close friends with some of his fellow diggers, especially Joachim Neumann, who was a consultant on the construction of the Channel Tunnel. "Some escape agents did it for money and were prepared to use force, which brought many of them into disrepute in the 1960s," Ms Nooke said. "But most of them worked very responsibly and were highly committed. Recognition for what they did has grown strongly in recent years."
For Mr Pfeifer, who recalled his extraordinary story in a matter-of-fact fashion, it was never about money or recognition. Asked if he still felt the hatred that drove him all those years ago, he said: "I wouldn't call it hatred any more. It's more a kind of contempt, for the Stasi people and the system they represented." email@example.com