The graveyard is truly empty now, Bobby thought. Save for that one small deep-buried cache — containing my most remote ancestors — these young rocks have given up all their layered dead.

And now a blanket of black cloud gathered, as if hurled across the sky by some impetuous god. An inverted rain began, rods of water that leapt from the dappled ocean surface to the swelling clouds.

A century wore by, and still the rain roared upward out of the ocean, its ferocity undiminished — indeed, so voluminous was the rain that soon ocean levels were dropping perceptibly. The clouds thickened further and the oceans dwindled, forming isolated brine pools in the lowest hollows of Earth’s battered, cracked surface.

It took two thousand years. The rain did not stop until the oceans had returned to the clouds, and the land was dry.

And the land began to fragment further.

Soon bright glowing cracks in the exposed land were widening, brightening, lava pulsing and flowing. At last there were only isolated islands left, shards of rock which shrivelled and melted, and a new ocean blanketed the Earth: an ocean of molten rock, hundreds of metres deep.

Now a new reversed rain began: a hideous storm of bright molten rock, leaping up from the land. The rock droplets joined the water clouds, so that the atmosphere became a hellish layer of glowing rock droplets and steam.

“Incredible,” David shouted. The Earth is collecting an atmosphere of rock vapour, forty or fifty kilometres thick, exerting hundreds of times the pressure of our air. The heat energy contained in it is stupendous… The planet’s cloud tops must be glowing. Earth is shining, a star of rock vapour.”

But the rock rain was drawing heat away from the battered land and — rapidly, within a few months — the land had cooled to solidity. Beneath a glowing sky, liquid water was beginning to form again, new oceans coalescing out of the cooling clouds. But the oceans were formed boiling, their surfaces in contact with rock vapour. And between the oceans, mountains formed, unmelting from puddles of slag.

And now a wall of light swept past Bobby, dragging after it a front of boiling clouds and steam in a burst of unimaginable violence. Bobby screamed -

David slowed their descent into time.

Earth was restored once again.

The blue-black oceans were calm. The sky, empty of cloud, was a greenish dome. The battered Moon was disturbingly huge, the Man’s face familiar to Bobby — save for a missing right eye… And there was a second sun, a glowing ball that outshone the Moon, with a tail that stretched across the sky.

“A green sky,” murmured David. “Strange. Methane, perhaps? But how…”

“What,” Bobby said, “the hell is that?”

“Oh, the comet? A real monster. The size of modern-day asteroids like Vesta or Pallas, perhaps five hundred kilometres across. A hundred thousand times the mass of the dinosaur killer.”

“The size of the Wormwood.”

“Yes. Remember that the Earth itself was formed from impacts, coalescing from a hail of planetesimals that orbited the young sun. The greatest impact of all was probably the collision with another young world that nearly cracked us open.”

“The impact that formed the Moon.”

“After that the surface became relatively stable — but still, the Earth was subject to immense impacts, tens or hundreds of them within a few hundred million years, a bombardment whose violence we can’t begin to imagine. The impact rate tailed off as the remnant planetesimals were soaked up by the planets, and there was a halcyon period of relative quiescence, lasting a few hundred million years… And then, this. Earth was unlucky to meet such a giant so late in the bombardment. An impact hot enough to boil the oceans, even melt the mountains.”

“But we survived,” Bobby said grimly.

“Yes. In our deep, hot niche.”

They fell down into the Earth once more, and Bobby was immersed in rock with his most distant ancestors, a scraping of thermophilic microbes.

He waited in darkness, as countless generations peeled back.

Then, in a blur, he saw light once more.

He was rising up some kind of shaft-like a well — toward a circle of green light, the sky of this alien, prebombardment Earth. The circle expanded until he was lifted into the light.

He had some trouble interpreting what he saw next.

He seemed to be inside a box of some glassy material. The ancestor must be here with him, one crude cell among millions subsisting in this container. The box was set on some form of stand, and from here, he could look out over -

“Oh, dear God,” said David.

It was a city.

Bobby glimpsed an archipelago of small volcanic islands, rising from the blue sea. But the islands had been linked by wide, flat bridges. On the land, low walls marked out geometrical forms — they looked like fields — but this was not a human landscape; the shapes of these fields seemed to be variants of hexagons. There were even buildings, low and rectangular, like airplane hangars. He glimpsed movement between the buildings, some kind of traffic, too distant to resolve.

And now something was moving toward him.

It looked like a trilobite, perhaps. A low segmented body that glittered under the green sky. Sets of legs — six or eight? — that flickered with movement. Something like a head at the front.

A head with a mouth that held a tool of gleaming metal.

The head was raised toward him. He tried to make out the eyes of this impossible creature. He felt as if he could reach out and touch that chitinous face, and — and the world imploded into darkness.

They were two old men who had spent too long in virtual reality, and the Search Engine had thrown them out Bobby, lying there stunned, thought it was probably a blessing.

He stood, stretched, rubbed his eyes.

He blundered through the Wormworks, its solidity and grime seeming unreal after the four-billion-year spectacle he had endured. He found a coffee drone, ordered two cups, gulped down a hot mouthful. Then, feeling somewhat restored to humanity, he returned to his brother. He held out the coffee until David — mouth open, eyes glazed — sat up to take it.

“The Sisyphans,” David murmured, his voice dry.

“What?”

“That’s what we must call them. They evolved on early Earth, in the interval of stability between the early and late bombardments. They were different from us… That methane sky. What could that have meant? Perhaps even their biochemistry was novel, based on sulphur compounds, or with ammonia as a solvent, or…” He grabbed Bobby’s arm. “And of course you understand that they need have had little in common with the creatures they selected for the cache. The cache of our ancestors. No more than we have with the exotic flora and fauna which still cling to the deep-sea vents in our world. But they — the thermophiles, our ancestors — were the best hope for survival…”

“David, slow down. What are you talking about?”

David looked at him, baffled. “Don’t you understand yet? They were intelligent. The Sisyphans. But they were doomed. They saw it coming, you see.”

“The great comet.”

“Yes. Just as we can see our own Wormwood. And they knew what it would do to their world: boil the oceans, even melt the rock for hundreds of metres down. You saw them. Their technology was primitive. They were a young species. They had no way to escape the planet, or outlive the impact themselves, or deflect the impactor. They were doomed, without recourse. And yet they did not succumb to despair.”

“They buried the cache — deep enough so the heat pulse couldn’t reach it.”