A cascade, yes, Malenfant thought. A chain reaction.

“And the water? What’s that for?”

“The emission of the Breath is associated with great heat — which is the point of the Engine. Water flows through the hillside, through the engine. The water is a cooling agent, which carries off this heat before any damage is done to the Engine. And the heat, of course, turns the water to steam, which in turn is harnessed to drive Mtesa’s various toy devices and fripperies…” Malenfant heard how de Bonneville’s voice slowed as he said that, as if some new idea was coming to him.

To Malenfant, it all made sense.

Twentieth-century nuclear-fission piles had been simple devices. They were just heaps of a radioactive material, such as uranium, into which reaction-controlling moderators, for example carbon rods, were thrust. Technical complexity only came if you cared about human safety: shields, robot devices to control the moderators, a waste-extraction process, and so forth. If you didn’t care about wasting human life, a reactor could be made much more simply.

With a little instruction, a tribe of Neandertals could operate a nuclear reactor. A bunch of children could. Especially if you didn’t care about safety.

“It’s the Breath that makes you ill,” he said.

“Indeed.”

“Why not others? Why not Mtesa himself?”

“The Breath is contained by the hundreds of meters of rock within which the Engine is housed. But, though it is not spoken of, there is much illness among the general population; and there are elaborate taboos about associating too closely with products of the Engine — you shouldn’t drink the water that has circulated through the yellow-cake, for instance.”

Malenfant remembered how Nemoto had warned him against inserting his hands in Mtesa’s fountains. He felt, now, a renewed itching in his own damaged skin.

Shit, he thought. I must have taken a dose myself.

De Bonneville waved his gnarled hands. “The Engine is clearly very ancient, Malenfant. The Waganda’s legend says it was constructed by an old king, seventeen generations before our own glorious Mtesa. It seems to me the Waganda have learned how to control their crude device, not by proceeding from a body of established knowledge as we might have done, but by trial and error over generations — and expensive trial and error at that. Expensive in human life, I mean!” But he was tiring, and losing interest. “Let me tell you of Mazuri…”

“You screwed the king’s favorite wife. You asshole, de Bonneville.”

“I tried to put her aside, when I left Rubaga to meet you. But when I returned, full of pombe and the excitement of the hunt, there she was… Ah, Malenfant, those eyes, that skin, that mouth…”

He was found out. Mtesa’s fury had been incandescent. De Bonneville was expelled from his position in court — dragged, by a rope around his neck, by Mtesa’s enthusiastic Lords of the Cord, and subjected to fifty blows with a stick, a punishment severe enough to lame him — and then banished to the lowliest position of Rubaga society: to work in the yellow-cake Engines, buried deep within the hillside.

De Bonneville grasped Malenfant’s arm with his ruined, clawlike hands. “It was all a trap, Malenfant. One accumulates enemies so easily in such a place as this! And I… I was always impetuous rather than careful… I was led into a trap, and I have been destroyed! Seeing you now, a traveler, makes me understand anew how much has been robbed from me by these savages of the future. But—”

“Yes?”

His blue eyes gleamed in his blackened ruin of a face. “But de Bonneville shall have his revenge, Malenfant. Oh, yes! His determination is sweet and pure…”

He confronted Nemoto.

“Nemoto, you know what the Engine is, don’t you? It’s a nuclear pile. A fucking nuclear pile.”

Nemoto shrugged. “It’s just a heap. Maybe a hundred tons of ‘yellow-cake’ — which is a uranium ore — with burned tree stumps used as graphite moderators. It was a geological accident: yellow-cake seams inside this hollow mountain, and some natural water stream running over the pile, cooling it…”

Natural nuclear reactors had formed in various places around the planet, where the geological conditions had been right. What was needed was a concentration of uranium ore, and then some kind of moderator. The function of the moderator was to slow down the neutrons, the heavy particles emitted by decaying uranium atoms. A slowed-down neutron would impact with another atomic nucleus and make that decay, and the neutron products of that event would initiate more decays — on and on, in the cascade of collapsing nuclei the physicists called a chain reaction.

Under Rubaga’s mountain, the action of water, over billions of years, had washed uranium ore from the rock and caused it to collect in seams at the bottom of a shallow sea. The uranium had then been overlaid by inert sand, and the rocks compressed and uplifted by tectonic forces, the uranium further concentrated by the slow rusting of surrounding rocks in the air. Thus had been created seams of uranium, great lenticular deposits, two or three meters thick and perhaps ten times as wide, under their feet, right here.

At first there had been no chain reaction. But then water and organic matter, seeping into cracks in the uranium seams, had served as primitive moderators, slowing the neutrons down sufficiently for the reaction process to start.

“The reaction probably started as a series of scattered fires in concentrations of the uranium ore,” Nemoto whispered. “Then it spread to less rich areas nearby. It was self-controlling; as the water was boiled by the reaction’s heat it would be forced out of the rock — and the reaction would be dampened, until more water seeped back from the surface layers above, and the reaction could begin again.” She smiled thinly. “And that is what the Neandertal community here discovered. It took them a couple of centuries, but they learned to tinker with the process, inserting burned wood — graphite — as secondary moderators…”

The workers in the pile maintained it with their bare hands. At times the workers had to haul heaps of yellow-cake from one part of the pile to another; or they mixed the yellow-cake, by hand, with other moderator compounds; or they cleared out the coolant-water pipes — the small fingers of children were well adapted for that particular chore. And as well as the regular operation of the pile, they had to cope with accidents, types of which Nemoto listed in the local language: leakages, spill-outs, crumbles, hot beds, slaps.

“Why did the Neandertals need to do this?”

“Because of us: Homo Sapiens, Malenfant. For a while, after the ice, the Earth was empty. The Gaijin implanted their little pockets of reconstructed prehumans. But then along we came, and it all unraveled as it had before, thirty or forty thousand years ago. You’ve seen how the locals treat the Uprights, the habilines.”

“Yes.”

“So it was with the Neandertals… except here. The Neandertals had their uranium, their radioactivity. They laced water supplies. They tipped their spears… It helped keep back the humans, until a smart human leader — a predecessor of Mtesa — came along and struck a deal.”

“So Mtesa supplies human slaves to the Neandertals. To maintain the pile.”

“Essentially. Makes you think, doesn’t it, Malenfant? If only the true Neandertals, of our own deep past, had discovered such a resource, perhaps they could have kept us at bay, survived into modern times — I mean, our times.”

Malenfant frowned. “It doesn’t sound too stable. A nuclear pile isn’t much of a weapon… You’d think that Mtesa’s soldiers could overwhelm the Neandertals, take what they wanted, drive them out. And the radioactivity — we’re all living on top of a raw nuke pile, here. Even those who don’t have to go work in that hole in the ground are going to suffer contamination.”