Callum MacRobert felt the first rumble on Christmas Day. No surprises there. It's always Christmas Day on MacRobert's World. This Christmas Day was in Lower Erbium, one of the spin-off keeps of the Rare Earths Association, not far from the plains of Ytterby. It happened during the morning carol service, just when the snow was lying round about "deep and crisp and even". If it had been in the afternoon, now, Callum might have put it down to too much brandy in the Christmas pudding. And if it had been him alone, he might have thought he'd stood up too quickly. But everyone felt it.

The ground rose and fell, like a long swell at sea, and dread swept through the congregation as long ago it swept through the Bethlehem shepherds as they stood before the Angel of the Lord. The singing faltered and stopped, and then everything was as it was. The ground was still. The barely audible subterranean sound, as of distant thunder, was gone. The instant dread turned into puzzlement. A sharp smell seemed to linger in Callum's nostrils, but perhaps that was the smoke from a guttering candle nearby.

"No real damage," the Seneschal of the Keep was saying. "Broken tableware, ornaments tossed off the shelves, that sort of thing. It was an earthquake all right, though."

"That's impossible," said Callum. He knew it wasn't, not entirely. But earthquakes weren't supposed to happen on MacRobert's World. In a way, they couldn't. Not real earthquakes. MacRobert's was an artificial suprajovian, a thin crust upon a hollow geosphere, supported in space and enclosing a massive gas-giant below. Still, even if tectonic activity was non-existent, the surface of the planet could be shaken.

"See if there's anything on the news," Callum suggested. "The Gypsy Elephant Moth XIV was due in orbit today." A multi-billion tonne starship smacking into the ground would shake any planet, even one as big as MacRobert's World. It might even punch right through, leaving the Habitat Authority to rush in with enormous gas bags and seal off the hole before the planet's atmosphere escaped. An emergency of that magnitude would be flashed on all channels.

"Nothing," said the Seneschal. "It must be local."

That wasn't impossible either. The Habitat Authority could raise or lower the surface over regions upwards of a hundred miles across; the geosphere base was flexible enough for that. With a major puncture they'd lift the entire surrounding landscape as high as sixty miles, like a vast shield volcano, cutting down on the air loss through the crater. A few feet was nothing. On the coast, that's how they made the tides. MacRobert's had no true moons, and Eta Cassiopeia, its soletta-magnified sun, was a long way in.

The Habitat Authority had been known to drown recalcitrant keeps by lowering the land about them below sea level. Or so the story went. Mind you, since keeps were usually megastructures several hundred metres high Callum didn't think the story quite held water, though he personally knew of several keeps rising directly out of the ocean. Bratsanger Keep, for one. The unimaginatively labelled Oceania for another.

Callum was pretty sure that no keeps contracted for earthquakes, and the Habitat Authority held itself strictly neutral in all MacRobert's numerous local conflicts. "Who've you upset?" he asked.

The Seneschal shrugged. "No one that I know of. Except the Christmas Rationalists, of course."

They both laughed. Anyone celebrating Christmas on a local calendar — and that meant almost everyone on MacRobert's World — upset the Rationalists, who demanded the imposition of a single official calendar instead and weren't above using violence to get it.

The next rumble was also on Christmas morning, over in Stanbeck-Moreton. Callum heard that it lasted several minutes and set fire to the tree in the refectory. The third rumble wrecked the midnight service at Nye Narvik and cast a pall over the Callum's festivities there the following day. No one could be sure when the next quake might strike.

Within a fortnight the pattern was clear. Quakes occurred only on Christmas Eve or Christmas Day, and, so far, only during carol singing. Something or someone didn't like Christmas. Or carols.

Quakes were localised to single keeps, converging in long Bessel-function waves from the area around them. Obviously not natural. Waves can spread out from a point of disturbance, but they can hardly spread in. There were no quakes as yet in the City, where diverse communities, with their disparate calendars, lived cheek by jowl. Callum guessed it might only be a matter of time, though.

Each quake was longer and stronger than the one before, not yet strong enough to cause structural damage to the keeps themselves, but already more than enough to terrify revellers and knock them off their feet. Holding a carol concert was becoming a real danger to life and limb.

"It's deliberate, of course," Lord Stavenger remarked to Callum, who'd dropped in for a chat with his librarian friend.

"Obviously," said Callum.

"The rumbles getting stronger, I mean. Everyone can see they'll carry on getting worse until people stop doing Christmas. Stonehouse has already cancelled theirs. Cowards."

Callum reflected that since Stavenger's Christmas was the same as Nye Narvik's Lord Stavenger now had almost a year's grace and could afford to take a robust line. Others weren't so fortunate. No one wanted to see his keep ripped apart.

"Unlikely," argued Miss Grey. "The geosphere would split first. Surely the fail-safes would cut in?"

Callum shrugged. "They should have done already. The Habitat Authority's going spare trying to work out how the actuators are being triggered. They've changed the access codes at least three times to my certain knowledge and technicians are only being allowed into the control rooms three at a time, with security guards on remote watch continually."

"How do they know it's Christmas, anyhow?" Miss Grey wondered.

"I suppose they've programmed a list of places and dates." As Christmas Specialist that was the sort of thing Callum kept track of all the time.

"No, I mean the carols. It's always carols."

"Listening to the vibrations coming through the geosphere, I'd guess. Or a plant in the congregation. If you're thinking of Christmas services without carols, I doubt it would work. It's all been planned too carefully."

Callum was right. The first deaths occurred when a massive quake struck the Ardent Quaker keep at Old Bethel Bethesda, which some thought unfair, since the Quakers considered Christmas unscriptural and music devilish, and treated the 25th of December as just another day. They did have a Sunday service in the chapel though.

As the toll mounted, so did the pressure to arrest or outlaw the Rationalists, or even the whole Christmas Unifier movement of which they were the fanatic wing. It had to be them, surely? Who else could the guilty parties be? There was little if any actual evidence, but not every keep was disposed to be particular. Some of the worst hit were understood to have interrogated suspects under drugs or torture — without result. If the Rationalists were indeed responsible, it seemed that only a tiny cabal was in the know.

The Rationalist stronghold at Hume-Sartre, where a dismal atheism ruled, was bombed flat by a consortium of new arrivals in whom the independent spirit was still strong. MacRobert's applauded. Or condemned. A rash of brush wars sprang up between those who approved of the bombing and those who insisted that even vermin like the Rationalists had a right to live. Oddly, the Christmas Unifiers, who disliked the Rationalists' materialism and religious intolerance but liked bureaucratically-imposed order, for the most part also favoured the crackdown — though there were some who speculated whether the quakes might not have been triggered by an anti-Unifier faction in order to discredit the Unifiers' cause.

Not every story has a tidy ending. The terrorists were never found. On Christmas Day, Home Calendar, the quakes stopped. They never started again. The rumble in the jumble was over. The Rationalists quietly disbanded. A year later the Resurgent Rationalists surfaced briefly, committed a few outrages, then sank back into oblivion. The Christmas Unifiers retrenched, but were otherwise barely affected.

Not for several months did the habitat engineers work out how the trick had been pulled. Normally, stability of MacRobert's planetary surface was maintained through feedback from the geosphere above the support actuators; the quakesters transmitted precisely calculated pressure waves from the multitudinous subterranean communications transducers, simulating geosphere movements the support system would automatically try to correct. These false signals triggered the quakes.

The solution to the puzzle brought new problems in its wake. Until new sensors could be installed and programmed to discriminate against spoofing — far from easy in so critical a system in which a single failure to correct a genuine displacement could be catastrophic — MacRobert's was vulnerable to a new sort of attack. Keeps could prosecute their customary conflicts by triggering earthquakes below the enemy. And so, of course, they did.

Inevitably, the Christmas Rumbles led directly to that period of MacRobert's history known as the Geo Wars, which was not ended until the Habitat Authority lost patience, hit Bradsnorsk-Dittersham with a keep-wrecking magnitude ten and flung it thirty thousand feet skywards, where most of the survivors suffocated in the thin air — then let it be known that from now on anyone else mucking around with the planetary support system would get more of the same.

As Callum said, "Christmas may come and Christmas may go, but smashing up the planetary crust when there's nothing but vacuum and gas giant below is bloody stupid."

© Paul Birch, 24th Dec. 2004.

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