Incandescence

Parantham said, "That surface looks as if it hasn't been repaired in fifty million years."

"Not everyone cares about surfaces," Rakesh replied. "There could still be someone home."

They sent a surveyor probe, which tomographed the artefact with ambient neutrinos. Inside was a maze of tunnels and caves. Apart from these empty spaces, there was an intricate pattern to the density of the structure itself: parts of the walls were solid as basalt, while others seemed as spongy and permeable as limestone.

Parantham beamed a radio signal from the surveyor probe down to the artefact, a simple message of greeting repeated across the frequency spectrum. The faint passive echo that came back suggested some long strips of conducting material, but no resonant circuits: electrical wiring, perhaps, but no obvious low-tech receivers or transmitters.

An analysis of the artefact's thermal emissions showed no significant amounts of heat being generated within, beyond what might be expected from a small amount of the fungus, and perhaps other species. There was no obvious stream of waste, organic or otherwise, leaving the artefact, though with the stellar wind as its only input any putative ecosystem would have to cling tightly to all of its materials.

Rakesh said, "It's time to send in the jelly babies."

"Ha! You were far more cautious with Steel Mountain," Parantham reminded him.

"If we trigger some elaborate defensive response," Rakesh said, "then at least I'll die happy. Knowing that this civilization survived."

There was no entranceway into the artefact, but the surveyor probe identified a system of narrow cracks in the exterior wall that ultimately led to one of the internal tunnels. If they made their avatars even smaller than before, about a fifth of a millimeter tall, they would be able to squeeze through.

Rakesh glanced up one last time into the sky full of hot blue stars before following Parantham into the chasm.

As the walls twisted around them, they soon reached a point where the stars were hidden and everything was swallowed by the deep shadows of vacuum; by switching to infrared vision, though, it was possible to grope their way down by the thermal glow of their surroundings. Their avatars sported adhesive pads on their hands and feet, tailored to the chemistry of the bare surface, but the infestations of fungus made their grip less secure.

Rakesh sent nanomachines from a stock in his avatar's arm into a patch of fungus, to sequence it. There were at least nine distinct species present, and they all showed marked differences from the kind found on the asteroids. The vestigial enzymes he'd noted there were being produced in far greater quantities here, and seemed to interact with several components of the wall material. As he pondered the modified diagram of metabolic pathways, he realized what was happening. The walls acted as a reservoir for the raw materials that the fungus needed, but the fungus did more than leech essential nutrients out of its environment. As part of its life cycle, it returned everything it took, with the added bonus that structural flaws in the wall were repaired in the process. The system wasn't perfect, but a few cracks after fifty million years wasn't bad.

It was a tortuous business navigating the fissure, but Rakesh wasn't tempted to disconnect his senses and leave his avatar on autopilot. He didn't know if he was entering a mouldy tomb or a thriving metropolis, but he had no wish to dilute, or distance himself from, the experience. As painful as he found it to be forced to confront the bleak prospects for life in the bulge, this expedition was exactly what he'd been seeking ever since he'd left his home world. Who else on Shab-e-Noor, who else in the whole disk, would be able to tell their descendants: "We climbed down through a gap in the wall, not knowing what we would find inside the structure after fifty million years"?

When they emerged into the tunnel, Rakesh found himself immersed in a featureless glow. The tunnel wall was so close to being uniform in temperature that its thermal emissions rendered everything in contrast-free monochrome. It was almost impossible to interpret what he saw, let alone navigate by it.

"Is it just me who's gone blind?" he asked Parantham.

"IR sensitivity isn't enough. We need to rewrite our whole visual processing system," she suggested.

Rakesh searched the library. Leaving aside olfactory and tactile modes — sniffing or groping your way through the dark — most underground species employed vibration sensors or sonar. The walls here were excellent sound conductors, but even so that would be of limited use. He found a mode of IR-based perception that some asteroid-mining robots and a few tunnel-dwelling species relied upon. It involved extracting and interpreting very small temperature differences from thermal emissions; it was exactly what he and Parantham needed.