Bruno had always known the caves were there, he wrote to Pascal and the Moulinards, but the depth of them, their spatial complexity, had stunned him.

We never expect the true depth of a cave, he said, on account of our indoctrination, our enslavement to the aboveground, which is scaled to us and above us, scaled to trees, to high-rise buildings, to the industrial dreams of twentieth-century man, and to his military imagination, scaled to fighter jets, and to heaven, to our need to claim something in the blue beyond, a thing we might call ‘blessed’.

This vertical arrow aiming from ground to sky constitutes modern man’s entire spatial reality, Bruno wrote. It excludes the other direction, he wrote, the down-into-the-earth. This is an incredible blind spot, he said, and he himself had not understood how blind, until he one day squeezed himself into his own cave, on his own little property in the Guyenne.

When he had purchased the land, in the early 1970s, the previous owner had shown him the cave as a curiosity. That owner had kept a board over its entrance. Beyond the board was an opening, a cavern five feet shallow, at the end of which two rocks angled together into a narrow crevice. For years, the board remained there. One day, in the period after he’d left the farmhouse, and the barn, and was sleeping in his little stone hut, Bruno removed the board and went in. He put his hand through the crevice in the rocks and felt wind. He understood that beyond the crevice there must be a large open space. He returned with ropes and a headlamp and pushed through the crevice and lowered himself. He did not hit bottom for quite a while. When he did, he was in an enormous room, its ceiling perhaps three meters high. He found multiple openings off this main room leading in different directions.

One particularly magnificent discovery was a chamber that was flocked white like a snowy landscape. Is this a dream? he wondered. It was not a dream. The walls were coated with magnesium crystals. They were blanketed in sparkling white, a natural geologic phenomenon. Some call this moon milk, Bruno wrote. It coated the floor. In that moon-milk floor were indentations that he believed were records of human presence, and in particular, shapes that looked, and felt, like the footprints of a child. There were regions of the underground network where water ran through, he said. The water was very cold. In some places it was neck-high, he said, and in the water lived strange crustaceans with translucent shells that seemed to thrive in absolute darkness.

This entire valley, he said, was laced with underground springs and rivers and lakes. Because Bruno’s adult son studied hydrology and currently worked in that field, he had helped Bruno to better understand the caverns and the water table and especially how to remain safe, because in winter, he said, caves could fill and quickly.

One day my son took me to a lavoir fed by a spring, Bruno said. The water is clear, my son pointed out. When the water is cloudy, he said, you know that someone has been in the cave whose spring fills this washbasin. The underground waterway, my son explained, has thick silt in its bed. When it is disturbed by footsteps, silt is kicked up.

The cave in which Bruno slept was dry year-round, if a good deal cooler in temperature than a modern Frenchman might prefer.

He planned to stay in the home he had made for himself underground. Although he did exit the caves regularly, as he indicated in the emails. He got fresh air. He tended to his permaculture, having renounced modern farming techniques. He took walks on shaded forest paths. And he wrote to Pascal Balmy and the Moulinards at a computer terminal that belonged to his adult daughter, in the kitchen of the old stone farmhouse. Bruno’s son had pointed out to him that the French government had more than clouded the waters of the communal washbasins. They had desecrated the entire subterranean world of southern France with tunnels for their high-speed trains.

I can hear the Paris–Toulouse from down there, Bruno said. I sense its vibrations. I feel the faintest touch of its wind.

Bruno’s son was of the opinion that the state’s mad plan to leach out all the groundwater and shunt it into industrial bays would wreck the ecological balance of the Guyenne.

When the digging work began for the Tayssac megabasin, Bruno said he was convinced he heard the sound of the excavators. He felt a level of disturbance that seemed to come from multiple directions, and from which there might be no escape. One day, the sound stopped, he said. The subterranean world was quiet, left in peace. But peace was temporary. These days, he said, I hear again the distant groan of machines clawing downward.

And just when I thought we were at last arriving at the main subject – sabotage of trains and of earth-moving equipment – Bruno went off the rails.