"So many worlds," Tris said.

Luca smiled.

Somewhere stacked in the back of her brain, Tris had Doc Joyce's breakdown of how and why. Obviously not the deep physics, the stuff that allowed each tesserae to retain its position within the globe while replicating gravity and retaining a workable atmosphere. All who claimed to understand this lied, their explanations quick and dirty hack around what little was left of preZP physics.

She learnt quickly, Tris was proud of this fact. Unfortunately the speed at which Tris assimilated ideas was something her grandmother never quite seemed to grasp. Her childhood refrain, You just never learn, do you?, being so far from the truth that Tris had seen little merit in pointing out that actually she learnt everything until there was nothing left to learn.

When Tris finally ran out of facts at home she went searching. No one ever thought to ask why and, if they had, Tris probably wouldn't have been able to answer. But the vanishing acts had grown in length, from missed afternoons through whole days to nights when she didn't come home and weeks that went by in a blur of cheap drugs, cheaper sex and bad conversation.

Translated, this meant reflex accelerators, fear inhibitors and a wide range of near opiates. Not to mention turning tricks against the wall at the back of Schwarzschilds for some tourist tom too blitzed to notice that Tris held him between her thighs instead of inside her.

The queens were cleaner, less animal, usually.

Many of those who put Tris up against a wall talked to her first, about their worlds and what made them come to one of the lowest levels of the Rip, a place most guides suggested they avoid. And once she even extracted a snatch of conversation from a gene splicer so silent even Doc Joyce had long since assumed the man was mute.

"Stop," said Luca and hands gripped her hips, halting Tris. "We're here."

Tris wanted to ask, where? There were so many things Luca assumed she knew when for most of this trip she'd merely been guessing. He was getting older and more tired, less happy to have her around. It was a look Tris knew well. One she'd seen each evening as a small child in the face of her mother, when the woman realized another day was gone and Tris's father had not returned. That, in all probability, he never would and she was left with a small child, a leaking shack and a mother-in-law who'd retreated into a world of her own.

And then one night, instead of looking resigned, Tris's mother had collected together the few things she actually owned and left. Tris wasn't even surprised.

"Keep staring ahead," said Luca. "You can't afford to turn round and you mustn't look down. Keep the moment close. And let everything else go."

Tris knew exactly what he meant. At least she hoped she did. "We're going to jump, right?"

"Not quite," said Luca, reaching into his satchel. For once the conditions were on their side. A whole strip of snow along the lip of the drop had slipped, exposing naked rock. This enabled Luca to find a flaw into which to ram the first of four steel pegs he produced from his bag.

"I'm going first," Luca told Tris. "And you're going to follow... The problem is I've only got a handful of these." He nodded to the spike. "So you're going to have to collect them as you go."

"How?"

"Easy," said Luca. "Just twist the top."

So Tris did, only too aware of the sheer drop towards which she shuffled, edging backwards so slowly she barely moved. Tris found the spike by dropping to a crouch and reaching behind her, fingers closing on cold metal.

It was stuck fast in the rock.

"Twist the top," Luca said again.

And Tris felt the spike slide free.

"We had hundreds once," said Luca, "thousands, maybe more." He sounded tired, old beyond his wish. "They were for building."

"You brought them with you?"

"It's possible," Luca admitted.

Holding the narrow spike in one hand, Tris twisted the top with her other.

"It's broken," she said.

Luca shook his head. Whatever was meant to happen took place at a level invisible to human eyes.

"It's working," he promised her.

She held a fortune in her hand, Tris realized, while another three fortunes lay at her feet. Doc Joyce would have restrung her entire body and thrown in new bones and buckytubes for her brain for one chance to work out what the spikes did and how.

Even fake tek was worth something. Certainly enough for the Doc to manufacture idiot-looking artifacts that tourists bought time and again, just in case they turned out to be real.

Tris grinned.

"What?" Luca said.

"Nothing that matters." Glancing back at the plateau, Tris was bemused both by the distance and the breath-catching beauty of a landscape she and Luca had crossed without really noticing. She was tired now. Almost as tired as Luca and the Baron was so tired that at times he seemed almost transparent.

"Focus," Luca said crossly. He nodded to the spike still gripped in her fingers. "And put that back."

Tris did, twisting the top to lock the spike in place.

"Right," said Luca. "Empty your head of everything but locating the next spike, reaching for it with one foot and letting the spike take your weight. You'll be roped to me and I'll be fixed to the cliff face with this." Luca pulled a final piece of climbing equipment from his bag. This spike had an eye at the top through which a rope could pass.

"One last thing," said the Baron. "You don't move until I tell you."

Tris understood that bit.

"A little to the right." Luca was doing his best not to sound worried. "Left a bit. That's it. The next spike's below your foot."

They'd been hanging on the edge of the drop for almost fifteen minutes and hardly made any progress at all. In fact, the lip over which they'd climbed was barely out of Tris's reach. All Tris had to do to follow Luca was remove the first and original spike, tuck it into her waistband and shift her weight so she could hang from a second spike, while using one foot to feel for a third that Luca had already fixed into the cliff.

"I know where it is," said Tris.

"Then use it."

Darkness was coming in faster than either had expected and Luca was running out of reassuring clichés about the first step being the most difficult, things getting easier, it just being a matter of practice...

"I thought you did this all the time in the Rip," Luca said, irritation winning out over tact.

"That's jumping," said Tris. "It's different."

Give her a rope long enough and she'd have been halfway down the cliff before Luca had finished fixing his wretched spikes.

"You must have climbed on Rip," said Luca.

"Of course I did," Tris said. "That was up, though. This is down..." All the same she twisted the spike, slid it from the rock and pushed it into the waistband of her thin trousers. She was climbing in her rope sandals, Luca having insisted that this would be better than bare toes.

"Well done," said Luca.

"Yeah," Tris said, "and you can fuck off too." But she said it too quietly for Luca to hear.