The rille was small — only twenty or thirty yards wide, its walls deep-cut. Henry had picked it out from old low-orbit Apollo photographs. In the low sunlight, with the regolith’s tan sparkle, its eroded walls looked like a small mountain valley, she thought, somewhere above the snow line.

“There,” said Henry. He pointed along the rille. “You see that?”

She looked where he pointed. A few hundred yards along, the rille terminated; but she could see a kind of bridge of rock beneath which the valley continued, as if it entered a tunnel.

“What is it?”

“A lava tube. Our salvation. I knew there had to be one here. Maybe we can live through this after all. Come on. We haven’t much time.”

This was Henry’s latest plan. She thought it was crazy. But she had to admit, now she’d slept on it, the idea of sacrificing her life without trying was less appealing than ever.

So, with Geena clinging on to Henry’s hand, watching they didn’t foul the tubing that joined them, they loped back to the Rover and began to unload it.

He fell inexorably from the empty lunar sky, every minute dropping five thousand feet and covering sixty more miles, the shadows lengthening as he rounded the curve of the Moon.

He must fly down the visible face of the Moon, all the way to the south, before landing. His altitude would drop steadily, sixty miles, forty, twenty, ten. He imagined his trajectory unwinding, a smooth curve shaped by gravity, kissing the surface of the Moon at just the point he intended, fifty miles short of the place he intended to deliver his nuclear weapon.

He was still flying at orbital speed — three thousand miles per hour, about Mach Five — and he would keep up those speeds, accelerate in fact, all the way to the surface of the Moon. Nobody in history had ever flown so fast, so low, not even Geena.

Certainly nobody had tried to achieve a touchdown at such speeds. And yet that was what he must attempt, today.

Through the tight portholes of his Soyuz, he caught glimpses of the surface of the Moon. It was a spotlit bombing range under a black sky, fleeing under his prow, fresh craters and basin rim mountains and undulating mare plains crowding over the close horizon with an unwelcome eagerness.

His view was completely sharp, of course. There was no cloud, no layer of muddy air, to obscure his view; and at times he would lose his sense of altitude. At such moments he turned away from the windows and trusted to his instruments, his infallible electronic senses, and to the precise mathematics that had guided him here.

And now, as Arkady flew further south, a new series of mountains — a ring of them, folded and eroded — came shouldering over the horizon towards him. They straddled the Moon, as if striving to block his further progress towards the Pole.

This was, he knew, the mountainous rim of the great South Pole-Aitken Basin: the huge impact crater which straddled the South Pole of the Moon, the largest and deepest such crater in the whole of the Solar System, a walled plain as wide as the Mediterranean Sea.

His Soyuz, like a little green bug, flew over the immense, eroded shoulders of the rim mountains. The mountains stretched before him and to either side, obviously ancient, colourless as plaster-of-paris models, a five-thousand-mile-long ring of shattered and folded Moonscape.

He checked his clock. Fourteen minutes to his touchdown. He was still seventy-five thousand feet high, with almost a quarter of the Moon’s face still to traverse; yet he was already inside the great Basin.

The land beyond the rim walls was revealed now. It was battered and scarred even beyond the norm he had come to expect for this small, ancient, rocky world, every square inch of it crowded with craters and rubble. The biggest craters here were major complexes in themselves, huge and eroded, many miles across, their giant flanks punctured by smaller, brighter newcomers.

In the shadows of the mountain ring, there were places where the sun could not have shone for a billion years — perhaps the coldest places, Arkady thought, in the Solar System.

It was there that Henry predicted water droplets from Moon-smashed comets would collect, snowing once into the shadows, and forever lying still. And it was there that Arkady must descend.

Ten minutes left. Fifty thousand feet: as high as he had flown, above Earth, before his first flight as a cosmonaut. And still he dropped, five thousand feet per minute, his descent as steady as ageing, and the fleeing Moon rose to meet him, as inexorable as death itself.

His controllers at Korolyov were silent. There was, it seemed, nothing more to say.

And now, as the land fled beneath him, at last — for the first time, and utterly unwelcome — he felt the brush of fear.

The lava tube was maybe ten yards wide. Its entrance was strewn with rubble, evidently cracked off of the roof. When she shone her helmet lamp into its depths, it extended further than her beam could reach.

“Good grief,” she said. “It’s long.”

Henry was moving into the tube, stepping carefully over the rubble-strewn floor, pulling their shared hoses behind him. To Geena, he was just a silhouette before the elliptical puddle of light cast by his helmet lamp.

She was forced to follow, reluctantly.

She was spooked by this place.

Of course she was being illogical. There could be nothing here to hurt her, not so much as a lunar rat. Nothing, in fact, had walked here since the tube’s formation, perhaps a billion years ago.

But even so…

“Look how sharp the rocks are,” Henry said. “No meteorite weathering here. Watch your step, Geena; this stuff could cut your suit to ribbons.”

“And you watch out for the damn hosepipe.”

“Yeah.”

“You know, the tube is bigger than I expected.”

“Well, this is the Moon. The last time I was in a lava tube was Hawaii. A couple of miles long… The lava on the Moon flows much more freely. That’s why you have the maria, great frozen puddles of the stuff. This tube might be ten miles long, maybe more.”

“Henry, help me with this damn shelter. If we don’t recharge our packs in the next couple of minutes, we’re screwed anyhow.”

“Yes, ma’am,” said Henry. He loped across to work with her.

When they had the shelter set up, they crawled inside.

They sat there in their EVA suits, fully pressurized, within their grimy inflatable shelter, sheltered by the rille lava tube, talking by torchlight.

Geena said, “How long before we see anything?”

“Maybe three hours after the detonation. The math is chancy.”

“That’s a long time to wait.”

“Not so long.”

“Tell me a story, Henry.”

“I don’t know any stories.”

“Tell me this is going to work. How much water is there at the South Pole?”

“I don’t know for sure. But the Moon is old, Geena. Enough time for a lot of volatiles to collect in the cold traps, where the sun never shines. Water and carbon dioxide. My models suggest there might be the equivalent of a thousandth the mass of a large cometary impactor, delivered by one process or another.”

“I’ll give you your thousandth,” she said. “So what?”

“Geena, a thousandth of a comet — if you melted it — could cover the Moon in water to a depth of a metre or so. A thousandth of a comet would contain enough carbon dioxide to form an atmosphere.”

The fans of her suit whirred patiently. She was sitting on a folded-up blanket, her suit stiff; now she came a little closer to him, as if seeking the human warmth trapped inside the layers of his suit. “So how come Prospector detected so little?”

“Because it’s logical. A working-out of the laws of physics. Solar System processes. It has to be there hidden in the deep regolith.”