Shellworlds themselves had accreted alternative names over the aeons: Shield Worlds, Hollow Worlds, Machine Worlds, Veil Worlds. Slaughter Worlds.
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.
In an Arithmetic Shellworld, the levels were regularly spaced at fourteen-hundred-kilometre intervals. Exponential or Incremental Shellworlds had more levels close to the core and fewer further out as the distance between each successive shell increased according to one of a handful of logarithm-based ratios. Arithmetic Shellworlds invariably held fifteen interior surfaces and were forty-five thousand kilometres in external diameter. Incremental Shellworlds, forming about twelve per cent of the surviving population, varied. The largest class was nearly eighty thousand kilometres across.
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.
Gases, fluids — especially water — and solids could be pumped or carried in to fill all or some of the spaces between the levels, and artificial interior ‘stars’ might be fashioned to hang from the ceilings of each level like gigantic lamps. The various venturesome species set about exploring the Shellworlds closest to them, and almost immediately encountered the problem that would bedevil, frustrate and delay the development of the worlds profoundly for the next few million years and, intermittently, beyond; the Shellworlds could be deadly.
It remained unclear to this day whether the defence mechanisms that kept killing the explorers and destroying their ships had been left behind by the worlds’ original builders or those who appeared to have dedicated their entire existence to the task of destroying the great artefacts, but whether it had been the Veil or the Iln — or, as it was now generally agreed, both — who had left this lethal legacy behind, the principal factor limiting the use of the Shellworlds as living spaces was simply the difficulty of making them safe.
Many people died developing the techniques by which a Shellworld might be so secured, and the same lessons generally had to be learned afresh by each competing civilisation, because the power and influence which accrued to a grouping capable of successful Shellworld exploitation meant that such techniques remained fiercely guarded secrets. It had taken an Altruist civilisation — exasperated and appalled at such a selfish waste of life — to come along, develop some of the techniques, steal others and then broadcast the whole to everybody else.
They had, of course, been roundly vilified for such unsporting behaviour. Nevertheless, their actions and stance had, in time, been ratified and even rewarded by various galactic bodies, and the Culture, although far remote in time from these now long-Sublimed people, had always claimed a sort of kinship by example with them.
The civilisations which specialised in making Shellworlds safe and who effectively took part-ownership of their interiors became known as Conducers. Sursamen was unusual in that two species — the Oct (who claimed direct descent from the long-departed Involucra and so also called themselves the Inheritors) and the Aultridia (a species with what might be termed a poorly perceived provenance) — had arrived at the same time and begun their work. It had also been unusual in that neither species ever got a decisive upper hand in the ensuing conflict which, in the only positive aspect of the dispute, at least remained localised to Sursamen. In time the situation within the world had been formalised when the two species were awarded joint protective custody of Sursamen’s access Towers by the then newly formed Galactic General Council, though, importantly, without any stipulation that the two could not contend for increased influence in the future.
The Nariscene were granted full inhabitory rights to the planet’s Surface and overall control of the world, formalising their long-held claim to it, though even they had to defer ultimately to the Morthanveld, in whose volume of influence the system and the world lay.
So Sursamen had been colonised, making it Inhabited, and by a variety of species, hence the Multiply- prefix. The holes in the supporting Towers that might have let gases or liquids vent to lower levels were sealed; some effectively permanently, others with lock complexes that permitted safe entry and exit, while transport mechanisms were installed inside the great hollow Towers to allow movement between the various levels and to and from the Surface. Material in gaseous, liquid and solid form had been moved in over the many millions of years of the planet’s occupation, and beings, peoples, species, species groups and whole ecosystems had been imported by the Oct and the Aultridia, usually for a consideration of some sort or another, sometimes at the behest of the peoples concerned, more often at the request of others.