Behind her, the traveltube chimed and the door rolled open. She turned and started walking towards it. ‘Well, come on, then!’ she said over her shoulder.

Phage Rock had been wandering the galaxy for nearly nine thousand years. That made it one of the Culture’s oldest elements. It had started out as a three-kilometre-long asteroid in a solar system which was one of the first explored by a species that would later form part of the Culture; it had been mined for metals, minerals and precious stones, then its great internal voids had been sealed against the vacuum and flooded with air, it had been spun to provide artificial gravity and it had become a habitat orbiting its parent sun.

Later, when the technology made it possible and the political conditions prevailing at the time made it advisable to quit that system, it had been fitted with fusion-powered steam rockets and ion engines to help propel it into interstellar space. Again due to those political conditions, it armed itself with up-rated signal lasers and a number of at least partially targetable mass launchers which doubled as rail guns. Some years later, scarred but intact, and finally accepted as personally sentient by its human inhabitants, it had been one of the first space-based entities to declare for the new pan-civilisational, pan-species grouping which was calling itself the Culture.

Over the years, decades, centuries and millennia that had followed, Phage had journeyed through the galaxy, wandering from system to system, concentrating on trading and manufacturing at first and then on a gradually more cultural, educatory role as the advances in technology the Culture was cultivating began to distribute the society’s productive capacity so evenly throughout its fabric that the ability to manufacture almost anything developed almost everywhere, and trade became relatively rare.

And Phage Rock - by now recognised as one of a distinct category of Culture artifacts which were neither ships nor worlds but something in between - had grown, accruing new bits of systemic or interstellar debris about it as its needs required and its population increased, securing the chunks of metal, rock, ice and compacted dust to its still gnarled outer surface in a slow process of acquisition, consumption and evolution, so that within just a millennium of its transition from mine to habitat its earlier, original self wouldn’t have recognised it; it was thirty kilometres long by then, not three, and only the front half of that initial body still peeped out from the prow of the knobbly collection of equipment-scattered mountains and expanded, balloon-like hangar and accommodation rotundae that now formed its roughly conical body.