My real fear was the border guards and Stasi, who were listening for movement beneath the ground and digging their own tunnels to intercept ours

In 1961, I was 21 and studying civil engineering in East Germany. I was desperate to escape to the west; the Berlin Wall, which started going up in August 1961, was becoming more impregnable every week. By winter, scores of people had been killed there or while trying to swim across the River Spree. It felt as if we were penned in, with no future at all.

I was lucky. I managed to cross into West Berlin using a smuggled Swiss passport. I had to leave my girlfriend, Christa, and family behind, thinking I would never see them again. I’d told my parents of my plan, but not Christa: I thought our relationship would be over. When I arrived in West Berlin, I had nothing but the clothes I was wearing. I got some state support as a refugee, and enrolled at another university, but I missed Christa and my family.

By now, border procedures had tightened up, and some school friends and I decided we would tunnel under the wall to help others get out. A friend put us in touch with other tunnel-digging students in need of manpower. We were part of a few successful projects; I had the skills to survey the tunnels, but other diggers were medical or philosophy students.

We would make a deep pit in the soil and, as a team, dig 24 hours a day, seven days a week, in 12-hour shifts over several months. Lying on our backs, we pushed the spade in 10cm each time, and used a wagon attached to a rope to remove the dirt. The tunnels were 140 metres long, 80cm high and 80cm wide – small enough to dig quickly but big enough to turn around in. While it was claustrophobic, my real fear was the border guards and Stasi, who were listening for movement beneath the ground and digging their own tunnels to intercept ours.

Some diggers were shot by the Stasi as they emerged in the east; one tunnel was compromised when a girl who hoped to escape was overheard by an informant. About 20 people were sent to prison for planning to escape through it. Christa was among them. I felt guilty, and angry, but there was nothing I could do.

Over the next few years, the diggers became anonymous heroes; we couldn’t tell anybody what we were doing, not even family. The most successful tunnel – later named Tunnel 57, for the number of people who escaped through it – was dug in 1964. It began in the cellar of a disused bakery and ended, by accident, in a disused outhouse in the garden of an East Berlin apartment building.

The day of the tunnel opening, I came home to find a letter from Christa telling me she had been released early, after 16 months in prison. I sent a courier to tell her about the tunnel, with a small teddy bear she’d sent me years earlier, so she’d know it was me.

That night, my job was to crawl through to East Berlin and stand in the outhouse to watch for escapees coming across the yard. They were families, couples – people we knew. The mood was quietly euphoric. Soon, a digger arrived with a woman in his arms. “Be careful with this girl,” he whispered, “she is so nervous.” It was Christa. We embraced, but I couldn’t go with her: I had to finish the job.

On the first night, 28 people escaped. By midnight on the second night, 29 others had come through. Then two unknown men appeared; they said they needed to collect their friend, waiting around the corner. They reappeared with a border guard. Shots were fired between a digger named Christian and the guard as we raced back through the tunnel.

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In the morning, East Berlin radio claimed agents from the west had killed a border officer. We were shocked. It was a propaganda coup for the GDR and the young guard, Egon Schultz, became a martyr. Christian went to his grave thinking he had killed a man; he never forgave himself. In the 90s, the truth emerged: Schultz had been killed by friendly fire from another guard.

Christa and I married and had a family; we lived in Frankfurt for 35 years, but I was blacklisted and couldn’t go with her and the children to visit their grandparents in the GDR. In 1971, I got a job with an international consulting company, responsible for transportation projects including the Channel tunnel. It’s my proudest achievement, but even that wasn’t without political difficulties: then, as now, some Brits wanted to remain an island.

Through my job, I got to see a world I could have only dreamed of while in the GDR. And thanks to the Berlin tunnels, many others did, too.

• As told to Nick Thompson

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