PHAROS WAS AN asteroid, part of the remains of a world that had been destroyed millions of years before. It hung in a broken necklace around a dead, blood-orange star. Across those asteroids were scattered mining colonies and hard-bitten missionary outposts; the system was almost completely forgotten.

A thousand years before, the Adeptus Mechanicus had followed their complex fate-equations and tech-priest divinations and arrived at the asteroid chain. They selected the region to be the seat of Stratix sector command, which in an emergency would serve to coordinate Adeptus Mechanicus troops, spacecraft and expertise. But information was most critical of all - the Adeptus Mechanicus was a priesthood, and its religion was knowledge. Information was the stuff of holiness, and the sector command had built a cathedral to learning that would hold all the information generated by the many adepts throughout the sector.

The cathedral jutted from the surface of one of the largest asteroids, the iron-heavy Pharos, bored out by giant tunnelling machines and plated in sacred metals - purest iron, solid carbon, bronze and zinc.

It took the form of a cluster of immense cylinders, arranged like the pipes of an organ, connected by thousands of glass bridges.

Several floors were below the surface of the asteroid, rooted into the super-dense core. Endless floors of knowledge and chapels of unfettered learning filled the cavernous spaces, and a regiment of combat servitors were hardwired into the structure to keep out the ignorant.

The delicate datastacks had to be kept cold to ensure their stability and the immutability of the information they contained. A whole ocean of liquid hydrogen flooded the lower levels, drowning the underground portion of the cathedral in the impossibly cold depths, fed by giant intakes that opened onto the asteroid's rocky surface. The captive ocean was regularly refilled by Mechanicus tanker craft in the never-ending cycle of holy maintenance that formed an act of worship for thousands of adepts and menials.

Inside, galleries of data-cubes were arranged above the freezing lake, almost alive with the immense volume of information they contained. A small body of tech-priests was permitted to live inside, sharing the cathedral with the maintenance servitors, bathing in the holiness of so much knowledge. They were adepts blessed for their devotion and service to the Machine-God with the opportunity to live out their extended lives in the icy splendour of Pharos.

When circumstances required, Pharos was a repository of vital knowledge that sector command could plumb for the good of the Imperium - the archives of its medical tower were at that moment being combed for solutions to the terrible plagues erupting throughout the Stratix warzone. But only tech-priests understood its real purpose - holy ground, created by the Mechanicus as a monument to the Omnissiah and a model of the Machine-God's ideal universe where immutable knowledge was the only reality.

There was no Chaos here, no evil randomness to pollute the sacred knowledge. And the Adeptus Mechanicus intended to keep it that way. No one was permitted access to Pharos except on the express order of the Archmagos Ultima, and he was known as a man not to be hurried. Only a handful of the Emperor's most trusted servants had been given access to the holy ground of Pharos, and then for the briefest periods of research under strict supervision. Some misguided souls and outright heretics had tried to force their way in, of course, but the holy ground had been kept inviolate with combat servitors and monitor ships.

No one had successfully stolen information from the cathedral of Pharos. But then again, no one had tried going in through the liquid hydrogen vents before.

'THE SEAL IS loose. Let me.' Lieutenant Kindarek reached over and adjusted the seal between Thad-deus's helmet and the neck ring of his hostile environment suit. Normally issued to explorator pioneers or engineers working on ships' hulls, the suit could keep extremes of temperature or noxious atmospheres from harming the occupant. All members of the recon platoon wore them, their faces appearing subtly warped through the square, transparent faceplates and their bulk increased by the thick, spongy dark grey material of the suits.

There was a hiss as the seal tightened and Thad-deus felt the air around his face turn cold and chemical. 'My thanks, lieutenant.'

The suited-up platoon was crammed into the rearward deployment airlock in front of Thaddeus, hellguns at the ready. Four storm troopers had grenade launchers slung on their backs and heavy garlands of frag grenade rounds looped on their belts. Neither the HE suits nor the fatigues underneath bore any Inquisitorial insignia, and Thaddeus himself didn't carry his Ordo Hereticus seal of authority - if the mission failed, there would be little to suggest that the Inquisition had been behind the infiltration.

None of them spoke. Thaddeus's own voice had sounded unwelcome and incongruous. How many battles had these men fought in? How many times had they waited in a Chimera APC or a Valkyrie airborne transport, not knowing if they would be dropped into a fire-fight?

Thaddeus knew several of these men had been at the Harrow Field Bridge where daemons of the Change God were emerging from the ground with the summer crops. Many had been part of the path-finding force that had found the tomb of the Arch-Idolator on Amethyst V. Some had scars and low-grade bionic eyes visible through their faceplates - all were silent and grim. Thaddeus's own nerves were tempered by his faith in the Imperial vision and the critical nature of the mission. Each man coped with the tense last few moments in their own way.

The shuttle tilted as the servitor-pilot in the cockpit turned it around. There was a metallic grating on the underside of the hull as the shuttle beached itself, braking jets pushing it up onto the shore.

'In position.’ came the servitor voice over the vox.

'Open us up.’ ordered Thaddeus. There was a squeal of hydraulics and the back wall of the passenger compartment dislocated, hissing downwards as the deployment ramp lowered.

Bright, cold, fluorescent light flooded in. The hydrogen lake filled the lower levels of this particular cylinder of the cathedral, and heaps of metallic cast formed piles under the surface that became sandbanks of silvery filings. It was against one of these that the shuttle beached. The beach glowed silver in the light and the ripples on the surface of the lake were as bright as knife blades.

The pointmen jumped onto the beach before the ramp was down, the huge boots of the HE suits splashing in the liquid hydrogen at the shore's edge. The photoreactive faceplates darkened in the glare as they panned the barrels of their hellguns over the area.

Kindarek's head tilted to the side for a moment as he received their voxes. 'All clear,' he voxed on his squad frequency. 'Move out.’

The platoon poured rapidly from the shuttle, boots kicking up the drifts of metallic shavings as they moved. Thaddeus followed, autopistol heavy in his hand and his mouth and nose already raw from the treated air. The light surrounded him as he stomped down the ramp onto the shore and he saw that the far wall was a single vast light source, phosphorescent gases trapped behind panes of transparent crystal, wrapping around the inside of the cylinder.

This cylinder of the cathedral was three kilometres across and perhaps ten high, with the lit section a hundred metres high. Access ladders wound their way in double helixes up to the first gallery levels. Columns hung from the distant ceiling, matt-grey so they drunk the abundant light. Between them were webs of glass walkways and platforms, thousands of filaments that turned the light flooding from below into a bright shimmering forest. It was like being inside a polished diamond, with a million faces looking up at the broken light of a new star. Clustered around the pillars and forming star-bursts of light at the intersections of the web were intricate crystalline sculptures in complex geometric shapes, mathematical prayers coded into the angles and faces, each sculpture a crystal information repository holding enough information to fill a hundred cogitator engines.