Still re-reading all of Iain M Banks.

Currently Matter

It's the one about Sursamen, the shellworld.

Sursamen collected adjectives the way ordinary planets collected moons. It was Arithmetic, it was Mottled, it was Disputed, it was Multiply Inhabited, it was Multi-million-year Safe, and it was Godded.

The Shellworlds had been built by a species called the Involucra, or Veil, the best part of a billion years earlier. All were in orbits around stable main-series suns, at varying distances from their star according to the disposition of the system’s naturally formed planets, though usually lying between two and five hundred million kilometres out. Long disused and fallen into disrepair, they had, with their stars, drifted out of their long-ago allotted positions. There had been about four thousand Shellworlds originally; 4096 was the commonly assumed exact number as it was a power of two and therefore – by general though not universal assent – as round a figure as figures ever got. No one really knew for sure, though. You couldn’t ask the builders, the Involucra, as they had disappeared less than a million years after they’d completed the last of the Shellworlds.

The colossal artificial planets had been spaced regularly about the outskirts of the galaxy, forming a dotted net round the great whirlpool of stars. Almost a billion years of gravitational swirling had scattered them seemingly randomly across and through the skies ever since: some had been ejected from the galaxy altogether while others had swung into the centre, some to stay there, some to be flung back out again and some to be swallowed by black holes, but using a decent dynamic star chart, you could feed in the current positions of those which were still extant, backtrack eight hundred million years and see where they had all started out. That four-thousand-plus figure had been reduced to a little over twelve hundred now, mostly because a species called the Iln had spent several million years destroying the Shellworlds wherever they could find them and nobody had been willing or able to prevent them. Exactly why, nobody was entirely sure and, again, the Iln were not around to ask; they too had vanished from the galactic stage, their only lasting monument a set of vast, slowly expanding debris clouds scattered throughout the galaxy and – where their devastation had been less than complete – Shellworlds that had been shattered and collapsed into barbed and fractured wrecks, shrunken compressed husks of what they had once been.

The Shellworlds were mostly hollow. Each had a solid metallic core fourteen hundred kilometres in diameter. Beyond that, a concentric succession of spherical shells, supported by over a million massive, gently tapering towers never less than fourteen hundred metres in diameter, layered out to the final Surface. Even the material they were made from had remained an enigma – to many of the galaxy’s Involved civilisations at least – for over half a billion years, before its properties were fully worked out. From the start, though, it had been obvious that it was immensely strong and completely opaque to all radiation.

They had been machines. In fact, they had all been part of the same vast mechanism. Their hollowness had been filled, or perhaps had been going to be filled (again, nobody could be certain this had actually been done), with some sort of exotic superfluid, turning each of them into a colossal field projector, with the aim, when they were all working in concert, of throwing a force field or shield round the entire galaxy.

Precisely why this had been thought necessary or even desirable was also unknown, though speculation on the matter had preoccupied scholars and experts over the aeons.

With their original builders gone, the people who had attacked the worlds seemingly also permanently missing and the fabled superfluid equally absent, leaving those vast internal spaces linked by the supporting Towers – themselves mostly hollow, though containing twisted webs of structurally reinforcing material, and punctured with portals of various sizes giving access to each of the levels – it had taken almost no time at all for a variety of enterprising species to work out that a derelict Shellworld would make a vast, ready-made and near-invulnerable habitat, after just a few relatively minor modifications.