You can buy this story as part of my collection, Valuable Humans in Transit and Other Stories.

A thousand years later, Humanity finally reached that magical point which had been anticipated since the dawn of mass media: the day when it was quicker, cheaper, safer and easier to go to the stars and look at them up close than to peer at them through telescopes.

It was then that the second, real Space Age began. Having long since built themselves perfect worlds on nearly every rock in the Sun system large enough to moor a habitat bubble, there remained nothing else in the universe more attractive to Humanity at large than the prospect of finally taking to the stars en masse. So it was that planet after planet orbiting star after star became home to new families of Humans, living under new skies and adapting to new environments, breeding new animals and looking at each other and the worlds around them with ever-new eyes. At last, the Human species was out of the cradle; at last, it no longer had a single point of failure. The future never seemed so expansive, the possibilities never so endless.

It was in the standard year 3198 that the ship Aspera Jaeyo set off around the curve of the galactic rim, far beyond the sphere of inhabited space, on an expedition to circumnavigate the galactic centre and map every step of the way. The mothership led a 500-strong fleet of smaller, crewed craft, connected invisibly to it by fine holeworm threads. The journey was planned to take what would have been, in briefer ages, a lifetime.

By thirteen years into the expedition, the crew of the Aspera Jaeyo had stellar cartography and astrometry refined to a fine art. As the Jaeyo bored its steady corkscrew route forward, its swarm of followers flitted from system to nearby system, deploying smaller autonomous drones to study each system's gas giants, terrestrial planets, comets, asteroids and parent star or stars. While the drones gathered and forwarded telemetry, the crewed craft swept onwards, typically seeding two or three dozen systems each in a standard day, but occasionally stopping to capture small comets for material to manufacture new drones, or returning to the mothership to exchange crew and undergo maintenance.

This was the routine which gradually came to be abandoned on the discovery of planet 0099-4836/010-D.

The anomalous data was received half a day after the subcraft Ulver had left the system, and it was another two hours before confirmation arrived from a second drone and the Ulver's subcaptain elected to backtrack and check the mysterious grey planet out. The drones' data proved accurate. Here was a planet, not substantially bigger than Sun/Earth, in a very Sun/Earth-like orbit around a very Sun-like single star, yet absolutely perfectly smooth — a blank slate, an oblate spheroid with an equatorial radius of 7,988 kilometres and a polar radius of 7,966, smooth to as many decimal places as the Ulver's instruments could determine. There was no trace of atmosphere, nor of variation in the pale grey colouring of its surface. There were no mountains, no tectonic plates, no plateaus, no geographical features, nothing.

There were no impact craters. At all.

A few explanations suggested themselves.

"Could it be a neutron star remnant? Smoothed down by billions of gravities?"

"Too large. And where is that gravity? Not enough density."

Odomad, the subcaptain, ordered a kinetic probe to be shot into the planet. Analysis of the debris propelled into space from the impact point showed the planet to be made mostly of something approaching magnesium steel, alloyed with a finely graded mixture of surprisingly complex molecules. To unpick the molecules' precise structure was not possible from space.

"Not neutron matter, then."

"And not strange quark matter..." (By this time, "strange" and "strange" were no longer the same word.)

"An egg?" Not everybody laughed at this.

Further kinetic probes fired at the planet's pole and equator returned similar data; seismic readings of the impacts' passage through the planet's interior suggested that it consisted of the same substance all the way through, or rather to the depth at which pressure forced it to become liquid.

Odomad gathered his subordinates together, and their reports, and mulled them. He synthesised their obvious but unspoken hunches together. He said,

"So, it could have been machined."

There was silence, as the crew considered the implied machinist, and their great, alien intent.

"A work of art? A religious icon?"

"A statement," someone said.

Odomad said, "I certainly feel as if someone has stated something to us."

*

He ordered a drone to land on the planet's surface and retrieve a sample. Contact was lost with the drone immediately after it landed.

The same happened with the second drone, even though it landed much more carefully than the first, using an entirely different method of propulsion which should have seen it land with less force than a feather.

The third drone was specifically configured to deliver extremely high-speed live internal telemetry at the instant of landing. A short time after it, too, became unresponsive, preliminary analysis of the telemetry revealed that the drone had been consumed by a sort of grey wave; consumed from the landing legs upwards to the radio dish in a matter of milliseconds.

Odomad turned, in response to a different alert. A holeworm thread was reeling in. The Aspera Jaeyo had arrived in the 0099-4836 system, having altered course to support the Ulver. Ten further subcraft arrived with it, along with countless drone ships.

The Ulver docked with its parent one astronomical unit above the star's north pole. Ekrem, the Jaeyo's captain, brought Odomad to the bridge, along with his data, and assistants. The bridge was windowless, but a holographic window showed a projection of the planet, featureless aside from the distorted reflection of its parent star, and cyan crosshairs where the various drones had interacted with it.

Ekrem gestured at the image. "So. We are looking at what, here?"

"You may remember the Blue-Age Mimas colony disaster from your history lessons," said Odomad, taking a sheaf of paper from his assistant and shuffling through the pages. "We're hypothesising that something very similar happened here, only on a vastly larger scale. We believe that at one point this planet had intelligent inhabitants. Their nanotechnology was advanced, significantly more advanced than ours is, even now. They, whoever they were, did what the Unknown Miman did. They created a nanobot with a very simple set of instructions: while one, propagate."

Ekrem was familiar with the principle. "But that doesn't work."

Odomad nodded. "No. To create a bot which can chew up all the processor substrate in a life support mainframe, as the Unknown Miman did, is easy. In that era, before we understood how to safeguard against it, it was a common enough accident, though Mimas was the most notable. But nanobots dead-end. They hit the same problems that organic bacterial life hits. Power generation and chemical decomposition, and environmental hostility beyond the rim of the Petri dish. Replication is hard. No simple eternal propagator can dominate a planet. It would require high-level intelligence."

"We couldn't do it," Ekrem said. "Even if we tried. We'd have to actively farm the stuff."

Odomad said, "We suspect that that could be exactly what happened. There are other explanations, and the circumstances surrounding this event are obscure at this point... Nevertheless, the nanofungus blankets the planet. And if that means that it had to be intelligently guided at a planetary scale, there is a strong possibility that this was intentional suicide, at a civilisational level."

"A dark possibility," Ekrem remarked.

Odomad reflected. "Dark, if measured on a Human baseline," he admitted. "Regardless of how it happened, it's happened. The nanofungus probably swept out the entire planet's surface over the course of months or years. Once everything on the surface was consumed, it had nowhere to go. So it obeyed its initial instructions and began to absorb the planet's atmosphere too, and to mine downwards as far as possible towards its core. The planet became smoothed over because this was the most efficient configuration. Then, once it reached the point where nothing more could be consumed, the fungus simply idled, waiting for more consumables to present themselves."

"Which explains the absence of impact craters."

"Exactly correct. Before you arrived, I arranged for a kilotonne asteroid to be dropped on the planet to see what would happen, we can show you that right now. This is sped up by a factor of thirty..."

A projected asteroid faded into being some distance above the projected planet, and fell in towards it under gravity. Ekrem took a few steps into the projection, squinting closely at the asteroid as it fell, then at the growing patch of bare grey metal where it was going to hit. Then he stood back to watch the impact.

"The asteroid itself takes several hours to be completely broken down, but the impact crater itself, you'll notice, is flattened much earlier in the timeline. We doped the asteroid with a radioisotope we can track from low orbit. We found that the new concentrations of elements brought by the impact were distributed evenly around the rest of the planet within hours. Which means there could have been a thousand similar impacts and we'd find no evidence of it. This planet could have been sitting here for hundreds of millions of years."

Ekrem turned from the hologram towards the live display of the fleet's in-system deployment. He took a few steps towards it, taking in the constellation of figures attached to each blip.

"Hundreds of millions of years?" he asked, turning around.

"Potentially billions," said the subcaptain. "We just don't know."

"A billion years of ravening, intelligent hunger," Ekrem mused. "Odomad, do we have the slightest idea what this planet could have been like... before?"

"In fact we... ah, I mean, yes, captain," one of Odomad's assistants said. "The rest of the system appears to be untouched by intelligent activity, which means we can apply the usual formation models. Assuming 010-D's mass hasn't significantly changed, we believe there would have been, at minimum, liquid water. Sulphur. Iron and magnesium. Methane. Potentially not oxygen... unfortunately it might take a significant number of years before we can hypothesise about how any actual life could have arisen..."

Ekrem's head snapped around again, suddenly. To one of his own people, he said: "Give me a detailed scan of the surface of this planet. I know we already have one, run it again. Find anomalies."

Time passed.

Ekrem turned to Odomad. "They were utterly alien to us," he asserted, quietly. "They must have been. But the rest of their system is untouched. Why? How much unlike us were they? Were they simply disinterested?"

"One way or another," Odomad said, uncertainly, "there was nothing left for them."

"Do we think they had eyes? Do we think, perhaps, that before... this... they lived all their lives under an impenetrable cloud?"

A young ensign named Beryt answered: "Three anomalies, captain. Moderate thruster activity and tentative holeworm drive signatures. Satellite scans show some sort of narrow pinnacles taking shape at the locations where... where each of the probes landed. They... they seem to be... rockets. They're taking shape incredibly quickly..."

Ekrem said, "Navigator, plot a direct course back to Sun. Straight path. No deviations. Do it right now. Engage the moment it's done. Maximum speed. Ruin the engine."

Odomad said, "Captain..."

Beryt cried, "We have ignition at the first landing site. The other two rockets have just disappeared but we're tracking their threads—"

Two red emergency lights lit up on the fleet graphic. Both were in close proximity to the grey planet. Four more in higher orbit lit up a second later, just as the first two faded to empty blue-edged squares, signifying lost contact. In an eyeblink twenty more were lit. That was the whole complement of in-system ships.

The Jaeyo was already a light-day out of the system and accelerating.

Something slug-like, dark and difficult for the Jaeyo's computer sensors to get a grip on, accelerated out of the blue-lit system behind them, and off at an angle, towards another nearby star. As the image of the system shrank behind them, more dark slug-things were detected heading in other directions, targeting other systems. Ships of the greater fleet began to appear at the edges of the display as the field of view widened. Red lights were already crawling over their scattered formations.

None of the ships, not even the probes, had weapons.

Obeying the emergency "Follow me" command issued manually by Ekrem, only a handful of the swiftest subcraft were able to escape the expanding maelstrom of hungry nanotechnology. They fell into a defensive pattern behind the Jaeyo, holding steady in its slipstream as it forged a superluminal path ahead. No signs of pursuit were detected, but could that be held to signify anything? Detecting a ship-sized object which didn't want to be found, even at a distance of mere light-seconds, was all but impossible. All ships and drones of the Jaeyo fleet carried beacons for precisely this reason.

Five days into the race home, the combined engineering forces of the surviving eight vessels succeeded in transplanting the Jaeyo's primary holeworm drive into the chassis of the lightest capable subcraft, the Twinarck. It would take an estimated six months to reach Sun, redlining all the way. Behind them, the sphere of influence of the nanofungus was already two hundred light years wide.

There was hope that Sun could be reached and Humanity at large warned before the first slugs arrived in Human space. But who knew what a million-year-old planet-sized brain could do? Who knew how fast it could build its fastest messengers, now that it knew what space travel was?