Just to be sure of his footing, he clambered back up the slope. There were no problems.

He turned, and bounded several yards back down the slope. Even from here he couldn’t see the bottom of the rille; it was still hidden by the broad shoulder of the valley.

The footing was good here. The Moon, shaped by impacts, was littered with rocky rubble. But this close to the edge, a lot of those fragments would have tumbled into the rille, carrying off the dust, and that had left the regolith here very thin—

Suddenly his left foot disappeared, out from under him.

“Shit.”

He looked down with surprise. His leg was just sinking into a pit of soft dust, softer and deeper than any other place he had come across, anywhere on the Moon.

He brought his right foot forward, but he caught it on a rock, and he fell with some force onto his hands and knees. His leg came loose of the dust pit, but his momentum rolled him forward, and the roll carried him over on his right-hand side.

“Henry. Are you okay?”

“Just a little soft here. I caught my… I stumbled over that rock.”

“You need me to help you?”

“No.”

He pushed his way to his feet, bouncing upright like a mannequin. He stepped back a little way from that patch of soft dust. It was a rough disc, he saw — and it swirled, gently.

Just as he’d seen in Scotland, on another planet, just as he’d predicted, he had found it here.

“Henry?”

“I’m fine.”

He carried on down the slope, following the tracks of Jays Malone as he had searched for his piece of lunar bedrock.

…And then he saw it, sheltering beneath a hummock in the regolith: an irregular-shaped crater in the regolith, scuffed by footprints. It was the original resting place of sample 86047, here on the Moon, a rock that had travelled a quarter-million miles from this spot, to his lab in Edinburgh.

He bent sideways, awkward in his suit, and ruffled the surface of the regolith with his fingertips, with reverence.

Then he straightened up. The tracks didn’t go any further, any deeper into the rille. The ghosts of Apollo couldn’t help him any more; and when he passed out of line of sight of Geena he wouldn’t be able to speak to her. Now, at last, he had to go where no human had travelled before.

He raised his gold sun-visor. For now, he wouldn’t need it.

Clutching his equipment, trailing the rope behind him, he marched on down the slope, into the shadow of the rille wall, alone.

It was dark here, the only light scattered from the far side of the rille.

He lit his helmet torch. It cast an ellipse of light on the regolith surface before him, and back-scattering illuminated a short way beyond that. He had to keep his head down so the light showed him where his feet would come down, so he could see only a few feet ahead of himself at any time.

He was truly, he thought, approaching the heart of darkness.

At first he moved cautiously. But the slope here, sandblasted by micrometeorite rain, was still gentle. Walkback limits, he thought; he could afford to go a little faster.

He lengthened his stride. Soon he was bounding in slow-motion leaps, sailing over the rocks and dust. When he landed he sent up sprays of dust which collapsed back immediately to the surface, like handfuls of gravel.

The trick with the light now was to keep it focused on the spot he was likely to land, rather than directly under his feet; it took some coordination, but he was getting the hang of it.

He allowed himself a moment of exhilaration. Hot shit, he thought; this is the way to do a field trip.

But now the ground was getting steeper, the regolith layer thinner; the sprays raised by his footfalls were diminishing.

He tried to slow. But he’d underestimated his momentum; old Sir Isaac was pushing at his back, and the surface under his feet was slithery. He leaned back and tried to dig his heels in, but that succeeded only in tipping him over backwards.

He slid six, ten feet on his butt, deeper into the rille. In his stiff Shuttle suit it was like sliding down a ski slope in a box.

He tumbled to a halt.

He sat there in darkness, breathing hard, his torch showing him only a few yards ahead, so close was the rille’s horizon now.

His rope gathered itself up, went taut, and he got a good hard tug on the chest. Geena, telling him to behave himself.

Well, she was right. It was going to be a lot harder coming back up.

He got up and slapped ineffectually at his thighs, trying to dust himself off. He held onto the rope, and stepped cautiously, ever deeper, his feet scraping now over almost bare basalt.

He came to a place where the slope was steeper than one in one. There was a basalt ledge here, a place where one of the lava strata had become exposed; it made a place to stand. When he looked up, his torch showed him the rille wall tipping up and away from him, dauntingly steep and tall.

He wasn’t sure he could go any further in. This would have to do.

He took his instrument bag off his shoulder, and placed it carefully at the back of the ledge. Then he got to his knees, and crept forward to the lip of the ledge. The suit fought him the whole way, but he crawled determinedly.

He stuck his head over the edge.

His torchlight splashed down over the steepening rille wall. More basalt layers. He inspected them briefly, noting their thickness, their difference in composition.

The whole history of mare volcanism was laid out here, clear as a road-map. He longed for a couple of grad students, a truck-load of sample bags. Maybe just by visual inspection, a couple of samples, he could achieve some good science here…

But he knew he mustn’t. His objective lay further on, and he must hurry to achieve it before his life support limits cut him short.

He lifted his head, and let his torch beam play further down the slope.

It looked as if the base of the rille, here, was no more than thirty or forty feet below him. The surface was smooth, silvery and flat.

It looked as if a river of some fluid had been dammed here.

Moonseed dust.

He raised his head further. His ellipse of light lengthened, until the glow became too attenuated to make out. As far as he could see, across the width of the rille, and to left and right along its length, the Moonseed dust lay, shimmering in his torchlight. It looked as if a river of mercury had come lapping through the rille’s dusty walls.

He found a loose rock, a fist-sized pebble an arm’s-length away. On impulse, he picked it up. He looked at the big astronaut’s Rolex on his wrist, set its timer, and dropped the rock.

He watched the rock fall, at first with dreamy slowness, and then with greater, more Earth-like velocity as the slow gravity had time to work. If he got the timing right he would be able to calculate pretty accurately how far below him that surface was…

In the light of his torch, the rock hit the surface, and disappeared. The Moonseed dust closed over it without a ripple, as if it had never existed.

He checked his watch. Four seconds. The Moonseed was forty-two, forty-three feet below him.

He knew he had to wait for the results of his seismic analysis. But already he knew what he would find out.

The whole damn Moon was rotten with Moonseed, just as he’d suspected.

And yet the Moon was still here, unlike Venus.

He crawled to the back of the ledge. He worked his way along the ridge, setting out his instruments, little tin-can boxes containing sensitive seismometers, shrouded in thermal blankets. The seismometer signals would be analysed as real-time data and in terms of frequency to record the ground’s vibration. He set up combinations of theodolites and electronic distance meters that might give some clue as to the deformation of the ground, even here on the side of the rille. And he deployed, close to the rim of his ledge, a small cospec, a correlation spectrometer, that would be able to measure emission rates of any gases it could detect. The instruments would be connected to a central data collection and communications unit and batteries by multiplex cables, that he plugged into the backs of the boxes.