His elder brother Roger, then 18, was killed June 23, 1944, when he fell down the hole. Venturing outside, Roger had stolen some machine gun ammunition from the Germans. They chased him back, shooting. He leaped for the rope, but let go as he slid down.
"Shots were crackling around the hole," said Mangnan. "He fell at my feet, behind me."
Finally, in July, they deemed it safe to climb out. Mangnan described the long scramble upward by ladder as "the fright of my life." "My mother said, 'don't look around, climb, climb, climb,' " he recalled.
They emerged to scenes of devastation, of hedgerows torn up and walls caved in.
Mangnan, his old memories tinged with almost boyish wonder, recalled seeing open boxes of artillery shells, jeeps and lorries rushing about, a Canadian field hospital, the burned shell of a German ammunition truck.
And his family home, its roof caved in.
In the postwar rebuilding of buildings and lives, the weeks underground tended to be pushed into Caen's collective subconscious, Simonnet said. Survivors preferred not to relive their wretched experiences, and historians concentrated on the battles and the soldiers who fought them rather than on the civilians' plight.
"People spoke about the landings and the liberators, but we haven't spoken until now of this daily life, living a little like hidden rats, in the humidity and dark, with death, pestilence and lice," he said.
"People brought their beds, their bikes, their mattresses, small pieces of wooden furniture. . . . There were births, deaths, underground hospitals, kitchens -- a real subterranean life that had been a bit underestimated until now."