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?”