"Here, take a look at this," Skellsgard said, directing Auger's attention to a knot of very close contour lines oozing into view on the display. "This is the computer's best guess at the shape of an approaching irregularity in the tunnel lining, based on the echoes from the sonar. If the contours were bunched together symmetrically, we'd be looking at a constriction, a narrowing in the tunnel ahead of us. But that isn't what's happening here. There are places where the tunnel lining looks as if it's been etched away, and places where it bulges inward. Theory one says that this is symptomatic of some kind of decay of the basic fabric of the link, either due to lack of maintenance or not enough ships using it."

"Not enough ships?"

"It could be that the ships are meant to perform some repair function when they pa.s.s through. That's what we call the 'pipe-cleaner hypothesis.'"

"Fine. What about theory two?"

"This is where it starts to get seriously speculative," Skellsgard warned. "Some people studying the link have made records of these irregularities, acc.u.mulating data from many transits. Of course, the data is very noisy and subject to the interpretive vagaries of the navigation system. So then they take those records and feed them into maximum-entropy software to squeeze out any latent structure. Then they take the output from that process and feed it into another bunch of programs designed to sniff out latent language. One such procedure is called the Zipf test: it involves plotting the logarithmic frequencies of the occurrence of different patterns seen in the walls. Random data has a Zipf slope of zero, whereas the Zipf slope of the tunnel patterning is pretty close to minus one. It means that the signals in those walls are significantly more meaningful than-say-squirrel-monkey calls, which only get down to minus point six on a Zipf plot."

"Not conclusive, though," Auger said.

"But the researchers don't stop there. There's another statistical property known as Shannon entropy, which even tells you how rich the communications are. Human languages-English, say, or Russian- have Shannon entropies around the eighth or ninth order. That means if I say eight or nine words in one of those languages, you can have a pretty good stab at guessing what the tenth is going to be. Dolphin calls have Shannon entropies in the range of three to four, whereas the tunnel scrawls are up at seven or eight."

"Less complex than human language, in that case."

"Granted," Skellsgard said, "but their true complexity might be masked by the errors we introduce in decoding the sonar images. Or the messages themselves may be blurred by erosion or some other process we don't understand."

"So theory two is that the patterns are deliberate messages."

"Yes. They might be a.n.a.logous to old highway signs: speed limits, temporary restrictions, that kind of thing."

"You're not serious."

"You haven't heard anything yet, Auger. Want to hear theory three?"

"Oh, why not?"

"This is definitely not accepted wisdom, I should warn you. Theory three says that the tunnel patterns

are a kind of advertising." Auger opened her mouth to say something, but Skellsgard kept on talking. "No, wait. Hear me out. It makes a warped kind of sense when you think about it. Why wouldn't a galactic supercivilisation have advertising? It seems to be pretty much glued to our culture, after all."

"But adverts..." Auger was finding it difficult to keep a straight face.

"Think about it. Anyone travelling along one of these links is the perfect captive audience. They're locked in, sucker bait. Got nowhere else to go, no other scenery to look at. What better place to put some

advertising? h.e.l.l, I'd love to know what they're selling. Maybe it's planet-building services, or stellar renewal, or the option to trade in your old black hole for a new one."

Auger smiled. "A supernova can happen any time. Make sure your solar system is properly insured."

"How about: tired of the Milky Way? Why not look at some of our great properties in the Large

Magellanic Clouds. The best views in the local group-and it's still within commuting distance of the

galactic core."

Auger chuckled, getting into it. "Expansionist primates infesting your stellar neighborhood? We have the pest-control solutions you need."

"Your old G.o.d not up to the job? Upgrade your deity now by calling..." Skellsgard started giggling.

"You're right-it's almost believable, isn't it?"

"Almost," Skellsgard said. "And I definitely prefer it to theory four."

"Which is?"

"That the walls are covered in graffiti."

"Goodness." Goodness. Had she really said "goodness?" Auger shook her head, like someone about to

sneeze. "Are you telling me that somebody's actually been paid to come up with that?"

"Yes. It even makes sense based on the Shannon entropies, apparently. If you look at human graffiti-"

"Enough, Skellsgard. I'd rather not hear about graffiti, human or alien."

"It's a bit depressing, isn't it?"

"More than a bit."

"Well, don't worry about it," Skellsgard said, waving a hand dismissively. "Not many people take it

very seriously. There's the small problem that the tunnel patterns have a habit of changing, depending on stability conditions. Of course, it might be very clever graffiti-"

"Is there a theory five?"

"Not yet. But I'm sure someone's working on one."

Auger laughed. Everything she knew about academia told her how true that was. Skellsgard's composure cracked as well, and it was only when they finished laughing, sighing with exhaustion and their eyes wet with tears, that Aveling opened his eyes and stared at them, his face as impa.s.sive as ever.

"Civilians."

In the twenty-ninth hour, something changed in the spiderweb crawl of Skellsgard's stress-energy display. The contours began to arrange themselves in a systematic and intricate pattern quite unlike the asymmetric bunching and stretching caused by the tunnel markings.

"You might want to look at this," she said.

"Is something wrong?" Auger asked.

"No. We're just coming up on something a little unusual, that's all. We always. .h.i.t it somewhere between the twenty-eighth and twenty-ninth hours, although it's never in quite the same place from trip to trip."

"More graffiti or tunnel turbulence?"

"Nope. Much too stable for that."

Auger leaned forward, relaxing her seat buckle. She kept her voice low. Aveling was asleep, snoring lightly, and she had no particular desire to wake him up. "So what are we looking at?"

"We're approaching a widening in the fabric of the tunnel. It's like a bubble, somewhat elongated in the direction of travel." Skellsgard made a few micro-adjustments to their flight path, signalled by a sequenced volley of steering jets. "At first, we didn't know what to make of it."

Auger tried to make some sense of the slowly moving contours, but she suspected it would need weeks of practice to untangle the information into anything approaching a three-dimensional image of their surroundings.

"And now?" she asked.

"We call it the 'interchange cavern,'" Skellsgard told her. "As far as we know, the Slashers have never found anything like this in any of their travels. All the connections they've mapped have been simple point-to-point affairs. You might get multiple cl.u.s.ters of portals located close to each other in s.p.a.ce, but you never get junctions in the hyperweb threads themselves."

"Except for this?"

"Well, there's obviously something special about this link because it feeds into the heart of an ALS. We think the interchange cavern allows selective access to different points in the crust of the captive planet." With one blunt fingernail she tapped particular features in the contour display. "There are nineteen possible routes out of the cavern, as far as we can tell, not counting the one we just arrived by. Trouble is, our steering control is only sophisticated enough to allow us to change course in time to reach six of the exits. Of the remaining thirteen, we've managed to drop lightweight instrument packages into four of them, but we never heard anything back. They probably didn't even make it to the ends of their threads."

"What about the six exits you can reach?"

"We always come out underground, within a few hundred metres of the surface. But five of the six exits are no use to us. Given time, we could tunnel our way to daylight, but it would take years, and every kilogram of rock we excavate would have to be brought back through the link."

"I'm missing something here," Auger said. "What's so difficult about digging through rock, given that you've already excavated half of Phobos?"

"There's a catch: our tools don't work on E2. We'd have to dig our way out with our fingers."

Auger asked the obvious question. "Wait. If you can't reach the surface, how do you even know it's the same planet? What if the threads lead somewhere else entirely?"

"Gravity's the main clue. It's always within a per cent or two of the same value, no matter where we pop out. Geochemistry varies a little, too, but not enough to lead us to think we're inside a different planet each time. We can plot these data points against our knowledge of E1 and take a stab at figuring out where we are-at least to within a continent's accuracy-but only one exit lets us reach the surface."

"Because it's closer?" Auger asked.

"No. Because there's another tunnel right next door. We only had to dig through a few dozen metres of actual rock before we hit a pre-existing shaft. If it wasn't for that..." Skellsgard's expression became philosophical. "Well, Susan would still be alive, and you'd still be looking at a tribunal."

"Thanks for the reminder."

"Sorry."

They pa.s.sed through the interchange cavern without incident. Less than an hour later, Aveling's sensors began to pick up the reflections from the approaching throat: the faint echo from the same kind of bow shock wave that had signalled the arrival of the other transport in the Phobos cavern. He told Skellsgard and Auger to secure themselves for arrival, which meant additional seat restraints and webbing, tightened to the point of discomfort. Auger recalled the violent arrival of the ship in Phobos and prepared herself for the worst.

When it came, it was mercifully quick, and she had no sooner registered the fact that the ship was slowing than she felt the arrestor cradle clang into position around the hull. The ship surged forward, halted and then lurched back as pistons took up the recoil. And then suddenly all was very calm, with Aveling reaching above his head to flick switches, powering down vital systems.

Auger had weight now, an unwelcome burden after thirty hours in free fall. It was an effort to move her arms to undo the seat harness, and a struggle to lift herself from the seat. Her muscles protested for a few moments as she began to stretch, and then, sullenly, resigned themselves to the task.

Presently, someone knocked on the door.

"That'll be Barton," Aveling said.

Barton turned out to be a younger version of Aveling, only with a slightly more enlightened att.i.tude towards civilians. He ushered them out of the transport, through a connecting airlock and into a rock-walled spherical cavern that was recognisable as a much smaller counterpart to the one at the Phobos end. Much equipment surrounded the recovery bubble, but there was no means to swap the existing transport for a refurbished one. Despite the damage it had sustained on the trip (light, Aveling said), the ship would simply be rotated through 180 degrees and sent on its way again.

Auger was introduced to two other people in the chamber: a tough-looking female military specialist called Ariano and another civilian technician called Rasht, a small, feline man with a sallow complexion. Neither of them looked like Slashers, and both appeared to have been working double shifts for at least a week.

"Any news on the others?" Aveling asked Ariano.

"Nothing," she said. "We're still transmitting on the usual frequencies, but n.o.body's called home."

Auger leaned against a red-painted handrail, unsteady on her feet. "What others?"

"Our other deep-penetration agents," Ariano said. "There are eight of them out there, some as far away as the United States. We've been sending out orders for them to return here."

"Because of what happened to White?"

"That's part of it. The link is also showing signs of instability, and we don't want anyone to end up marooned here."

"This is the first I've heard about any instability," Auger said uneasily.

"It'll hold long enough for you to complete your mission," Skellsgard replied.

"We're also concerned about the political situation at home," Ariano said. "We know things are hotting up back there, and that some people are talking about a Slasher invasion. If they're right, there's a danger we'll lose Phobos. We can't afford to have anyone still here if that happens."

"All the more incentive to get things done as quickly as possible," Aveling said. He clicked his fingers at Ariano and Rasht. "Get the ship prepped for the return leg. I take it you have cargo?"

Rasht was standing next to an incongruous-looking tower of cardboard boxes. The topmost box was crammed with books, magazines, newspapers and gramophone records. "Five hundred kilograms' worth. A few more trips and we'll have sent home everything Susan delivered."

"Good," Aveling said. "Get it loaded and secured. You can ship out as soon as you're ready."

"Wait," Auger said. "Is that ship leaving without me?"

"There'll be another one back sixty hours after this one departs," Aveling said, his voice unctuous with sarcastic sweetness. "That gives you at least two and a half days to complete your mission. If you get back with the tin sooner than that, you can simply sit tight here and wait for the next transport."

"I still don't like the idea-"

"This is the way it's going to happen, Auger, so deal with it," Aveling said bluntly, terminating the conversation by turning away.

The three of them trooped off the catwalk, leaving Barton, Ariano and Rasht to load the transport for its return flight. They reached a circular deck surrounding the chamber. Prefabricated cubicles ringed the deck, along with equipment lockers and control consoles. In the deep pit below the bubble, powerful generators snored to themselves, umbilicals snaking across the floor like draped tentacles.