“You said there was a math for this?”

“So I’m told.”

“You don’t know it?”

“It’s not twenty-second-century math. It’s several millennia beyond that. I doubt you or I could contain it without a certain amount of neural augmentation.”

Catherine said, “This is awfully abstract.”

Archer nodded and seemed to struggle a moment with his thoughts.

Ben looked out the window. There was something wonderfully calming about all these Douglas firs. The sound they made when the wind moved through them.

Archer cleared his throat. “There’s another obvious question.”

The painful question. “You want to know what went wrong.”

Archer nodded.

Ben sighed and took a breath. He didn’t relish these memories.

He had reconstructed this from his own experience, from the fragmentary memories of the cybernetics, from the evidence of the tunnel itself.

There was a house like this house, he told Archer and Catherine, a temporal depot, in the latter half of the twenty-first century, in Florida—in those days a landscape of fierce tropical storms and civil war.

The custodian of that house was a woman named Ann Heath.

(Ann, he thought, I’m sorry this had to happen. You were kind when you recruited me and I never had a chance to repay that kindness. Time may be traversed but never mastered: the unexpected happens and in the long run we are all mortal.)

The Florida house had been scheduled for shutdown. Its environment was growing too unpredictable. But something unexpected happened prior to that closing. As nearly as Ben could deduce from the available clues, the house had been invaded by forces of the American government.

The house had possessed some defenses and so did Ann Heath, but perhaps these had been partially dismantled prior to shutdown; in any case, the soldiers of the grim last decades of that century were formidable indeed, with weapons and armor rooted deep into their bodies and nervous systems.

One of these men must have occupied the house, overpowered Ann, and forced her to reveal some of the secrets of the tunnel. The man had used this information to escape into the past.

(She must he dead, Ben thought. They must have killed her.) The marauder had invaded Ben’s domain without warning, disabled the cybernetics with an electromagnetic pulse, destroyed much of Ben’s body, and dumped his corpse in the woodshed. The attack had been quick and successful.

Then the marauder had opened a tunnel some thirty years long, to a nodal point in New York City, where he had committed the same sort of attack but more thoroughly; another custodian and all his cybernetics were irretrievably destroyed.

Finally—as a last, shrewd defense—the marauder had disabled the tunnel’s controls so that the connection between Belltower and Manhattan was permanently open.

Catherine said, “Permanently open? Why is that such a great idea?”

Ben was lost a moment in temporal heuristics, then hit on a simple analogy: “Imagine the nodal points as terminals in a telephone network. Simultaneous connections are impossible. I can call a great number of destinations from one phone— but only one at a time. As long as the connection with Manhattan is open, no other connection can be made.”

“The phone is off the hook,” Catherine said, “at both ends.”

“Exactly. He’s sealed himself off. And us along with him.”

“But a phone,” Catherine said, “if it doesn’t work, you can always go knock on the door. Somebody from another terminal somewhere else could have shown up and helped. Better yet, they could warn you. Leave a message in 1962: In seventeen years, watch out for a bad guy.”

Oh dear, Ben thought. “I don’t want to get too deeply into fractal logistics, but it doesn’t work like that. Look at it from the perspective of the deep future. Our time travelers own a single doorway; its duration governs duration in all the tunnels. From their point of view, Belltower 1979 and Manhattan 1952 disappeared simultaneously. Since that disappearance, approximately ten years have elapsed—here, and in the New York terminus, and in the future. And there are no overlapping destinations. The portal in this house was created in 1964, twenty-five years ago, when its valency point with Manhattan was the year 1937 … Are you following any of this?”

Catherine looked dazed. Archer said, “I think so … but you could still leave a message, seems to me. A warning of some kind.”

“Conceivably. But the time travelers wouldn’t, and the custodians have sworn not to. It would create a direct causal loop, possibly shutting down both terminals permanently.”

“ ‘Possibly’?”

“No one really knows,” Ben said. “The math is disturbing. No one wants to find out.”

Archer shrugged: he didn’t understand this, Ben interpreted, but he would take it on faith. “That’s why nobody came to help. That’s why the house was empty.”

“Yes.”

“But you survived.”

“The cybernetics rebuilt me. It was a long process.” He gestured at the stump of his leg under the blanket. “Not quite finished.”

Catherine said, “You were out there for ten years?”

“I wasn’t suffering, Catherine. I woke out of a long sleep, the day you opened the door.”

“Then how do you know all this?”

This was easier to demonstrate than explain. He made a silent request and one of the cybernetics climbed the bed-sheets and sat a moment in the palm of his hand—a glittering, many-legged jewel.

“My memory,” he said.

“Oh,” Catherine said. “I see.”

This was an awful lot to accept all at once, Archer thought. Time as a fragmented structure, like sandstone, riddled with crevices and caverns; twenty-first-century marauders; insect memories …

But Ben made it plausible. Plausible not because of his exoticisms—his strange injuries or his tiny robots—but because of his manner. Archer had no trouble at all believing this guy as a twenty-second-century academic recruited into an odd and secret business. Ben was calm, intelligent, and inspired trust. This could, of course, be a clever disguise. Maybe he was a Martian fifth columnist out to sabotage the planet—given recent events, it wouldn’t be too surprising. But Archer’s instinct was to trust the man.

Questions remained, however.

“Couple of things,” Archer said. “If your marauder did such a thorough job at the Manhattan end, why did he screw up here?”

“He must have believed I was dead beyond reclamation. Probably he thought all the cybernetics were dead, too.”

“Why not come back and check on that?”

“I don’t know,” Ben said. “But he may have been afraid of the tunnel.”

“Why would he be?”

For the first time, Ben hesitated. “There are other … presences there,” he said.

Archer wasn’t sure he liked the sound of this. Presences? “I thought you said nobody could get through.”

The time traveler paused, as if trying to assemble an answer.

“Time is a vastness,” he said finally. “We tend to underestimate it. Think about the people who opened these tunnels— millennia in the future. That’s an almost inconceivable landscape of time. But history didn’t begin with them and it certainly didn’t end with them. The fact is, when they created these passages they found them already inhabited.”

“Inhabited by what?”

“Apparitions. Creatures who appear without warning, vanish without any apparent destination. Creatures not altogether material in constitution.”

“From an even farther future,” Archer said. “Is that what you mean?”

“Presumably. But no one really knows.”

“Are they human? In any sense at all?”

“Doug, I don’t know. I’ve heard speculation. They might be our ultimate heirs. Or something unrelated to us. They might exist—somehow; I find it difficult to imagine—outside our customary time and space. They seem to appear capriciously, but they may have some purpose, though no one knows what it is. Maybe they’re the world’s last anthropologists—collecting human history in some unimaginable sense. Or controlling it. Creating it.” He shrugged. “Ultimately, they’re indecipherable.”