But this was why they had launched the revolt when they had. On Mars, the independence movements had most of the cities under their control. Sheffield was still a metanational stronghold, but Peter Clayborne was up there, in command of all the insurgents on Pavonis, coordinating their activities in a way that they had not been able to match around Burroughs. Partly this was because many of the most radical elements of the resistance had avoided Tharsis, and partly because the situation in Sheffield was extremely difficult, with little room for maneuvering. The insurgents now controlled Arsia and Ascraeus, and the little scientific station in Crater Zp on Olympus Mons; and they even had control of most of Sheffield town. But the elevator socket, and the whole quarter of the city surrounding it, were firmly in the hands of the security police, and they were heavily armed. So Peter had his hands full on Tharsis, and would not be able to help them around Burroughs. Nadia talked to him briefly, describing the situation in Burroughs and begging him to call Ann and ask her to get the Reds to show some restraint. He promised to do what he could, but did not seem confident that he had his mother’s ear.

After that Nadia tried another call to Ann, but did not get through. Then she tried to reach Hastings, and he took her call, but it was not a productive exchange. Hastings was no longer anything like the complacent disgusted figure she had talked to the night before. “This occupation of the dike!” he exclaimed angrily. “What are they trying to prove? Do you think I believe that they’ll cut the dike when there’s two hundred thousand people in this city, most of them on your side? It’s absurd! But you listen to me, there are people in this organization who don’t like the danger it puts the population in! I tell you, I can’t be responsible for what’ happens if those people don’t get the hell off that dike — off Isidis Planitia entirely! You get them off there!”

And he cut the connection before Nadia could even reply, distracted by someone off-screen who had come in during the middle of his tirade. A frightened man, Nadia thought, feeling the iron walnut tugging inward again. A man who no longer felt in control of the situation. An accurate assessment, no doubt. But she had not liked that last look on his face. She even tried to call back, but . no one in Table Mountain would answer anymore.

A couple of hours later Sax woke her up in her chair, and she found out what Hastings had been so worried about. “The UNTA unit that burned Sabishii went out in armored cars and tried to — to take the dike away from the Reds,” Sax told her, looking grave.

“Apparently there’s been a fight over the section of the dike nearest the city. And we’ve just heard from some Red units up there that the dike has been broached.”

“What?”

“Blown up. They had drilled holes and set charges to use as a — as a threat. And in the fighting they ended up setting them off. That’s what they said.”

“Oh my God.” Her drowsiness was gone in a flash, blown away in her own internal explosion, a great blast of adrenaline racing all through her. “Have you got any confirmation?”

“We can see a dustcloud blocking the stars. A big one.”

“Oh my God.” She went to the nearest screen, her heart thudding in her chest. It was three A.M. “Is there a chance ice will choke the gap, and serve as a dam?”

Sax squinted. “I don’t think so. Depends on how big the gap is.”

“Can we set counterexplosions and close the gap?”

“I don’t think so. Look, here’s video sent from some Reds south of the break on the dike.” He pointed at a screen, which displayed an IR image with black to the left and blackish green to the right, and a forest-green spill across the middle. “That’s the blast zone there in the middle, wanner than the regolith. The explosion appears to have been set next to a pod of liquid water. Or else there was an explosion set to liquefy the ice behind the break. Anyway, that’s a lot of water coming through. And that will widen the break. No, we’ve got a problem.”

“Sax,” she exclaimed, and held on to his shoulder as she stared at the screen. “The people in Burroughs, what are they supposed to do? God damn it, what could Ann be thinking!”

“It might not have been Ann.”

“Ann or any of the Reds!”

“They were attacked. It could have been an accident. Or someone on the dike must have thought they were going to get forced away from the explosives. In which case it was a use-it-or-lose-it situation.” He shook his head. “Those are always bad.”

“Damn them.” Nadia shook her head hard, trying to clear it. “We have to do something!” She thought frantically. “Are the mesa tops high enough to stay above the flood?”

“For a while. But Burroughs is at about the lowest point in that little depression. That’s why it was sited there. Because the sides of the bowl gave it long horizons. No. The mesa tops will get covered too. I can’t be sure how long it will take, because I’m not sure of the flow rate. But let’s see, the volume to be filled is about …” He tapped away madly, but his eyes were blank, and suddenly Nadia saw that there was another part of his mind doing the calculation faster than the AI, a gestalt envisioning of the situation, staring at infinity, shaking his head back and forth like a blind man. “It could be pretty fast,” he whispered before he was done typing. “If the melt pod is big enough.”

“We have to assume it is.”

He nodded.

They sat there beside each other, staring at Sax’s AI.

Sax said hesitantly, “When I was working in Da Vinci, I tried to think out the possible scenarios. The shapes of things to come. You know? And I worried that something like this might happen. Broken cities. Tents, I thought it would be. Or fires.”

“Yes?” Nadia said, looking at him.

“I thought of an experiment — a plan.”

“Tell me,” Nadia said evenly.

But Sax was reading what looked like a weather update, which had just appeared over the figures scrolling on his screen. Nadia patiently waited him out, and when he looked up from his AI again, she said, “Well?”

“There’s a high-pressure cell, coming down Syrtis from Xanthe. It should be here today. Tomorrow. On Isidis Planitia the air pressure will be about three hundred and forty millibars, with roughly forty-five percent nitrogen, forty percent oxygen, and fifteen percent carbon diox—”

“Sax, I don’t care about the weather!”

“It’s breathable,” he said. He eyed her with that reptile expression of his, like a lizard or a dragon, or some cold posthuman creature, fit to inhabit the vacuum. “Almost breathable. If you filter the CO₂. And we can do that. We manufactured face-masks in Da Vinci. They’re made from a zirconium alloy lattice. It’s simple. CO₂ molecules are bigger than oxygen or nitrogen molecules, so we made a molecular sieve filter. It’s an active filter too, in that there’s a piezoelectric layer, and the charge generated when the material bends during inhalation and exhalation — powers an active transfer of oxygen through the filter.”

“What about dust?” Nadia said.

“It’s a set of filters, graded by size. First it stops dust, then fines, then CO₂.” He looked up at Nadia. “I just thought people might need to get out of a city. So we made half a million of them. Strap the masks on. The edges are sticky polymer, they stick to skin. Then breathe the open air. Very simple.”

“So we evacuate Burroughs.”

“I don’t see any alternative. We can’t get that many people out by train or air fast enough. But we can walk.”

“But walk to where?”

“To Libya Station.”

“Sax, it’s about seventy k from Burroughs to Libya Station, isn’t it?”

“Seventy-three kilometers.”

“That’s a hell of a long way to walk!”

“I think most people could manage it if they had to,” he said. “And those who can’t could be picked up by rovers or dirigibles. Then as people get to Libya Station, they can leave by train. Or dirigible. And the station will hold maybe twenty thousand at a time. If you jam them in.”