‘As you know, the authorities built the Wall between East and West Berlin in 1961, which ended up approximately a hundred miles long. This tunnel was begun in 1964 when the attempts to tunnel were at their peak. It is unusual in that it was dug from West to East, not the other way around.’

Foetid air with no life in it seeped up from underground.

‘Cold War tunnels have a special place in Berlin’s history,’ said the guide who did not look especially poetic or empathetic. ‘They’re symbols of survival and a willingness not to give up.’

Petr dug his hands into his coat pockets. For a communist, ex or otherwise, the subject was uncomfortable. For the nth time, he reflected on history’s contortions. ‘Not so long ago it was the Third Reich who was the oppressor,’ he said into Laure’s ear.

‘Communist, fascist… same difference,’ she whispered back.

The guide asked them to gather around. ‘There are several things to remember if you are planning on digging a clandestine tunnel under a wall…’ Something approximating humour was discernible on the broad features. ‘First, once you have hit the water table stop digging downwards and go forwards instead. Two, operate with a password system otherwise you won’t know if the group has been penetrated. Three, a screwdriver will melt if it touches the electric grid. Four, to stop guards becoming suspicious it is necessary to live on the site. Five, if you are going to dig, it’s best to choose a nice, light sandy soil like we have in Berlin.’

Petr glanced around. At 3.6 metres, the height of the Wall may have obscured some comings and goings, but Stasi guards would have been mounted in the buildings and the watchtowers. Digging and constructing a tunnel would have taken nerves of the first order – and probably an engineer.

Normally, he refused to put himself into others’ shoes as it weakened resolve. But now he could not help thinking what a fearful, dangerous business it must have been. An undertaking that would rip nerves to shreds. He glanced at Laure. She looked appalled, as if she couldn’t begin to imagine what it took to drive someone to escape through a crumbling shaft.

He shifted up to stand beside her. ‘It’s always worse thinking about it than doing it,’ he said.

‘It’s a thought to cling onto,’ she replied, with barely disguised bitterness.

The guide continued: ‘We gather that the group who dug this tunnel lived on site for over five months, sleeping here in week-long shifts, using buckets of water to keep themselves clean. It was a heroic achievement. Although they had no idea where they were going to emerge, they kept on digging.’ He paused for effect. ‘Luck was on their side and they found themselves in a disused lavatory behind an apartment building.’

Petr could imagine. Stench worse than dead bodies. Filth.