She forced herself to focus, to see. The near edge of the flood was about a kilometer away, and two hundred meters below them. The foot of the northern wall of Ius was about fifteen kilometers away, and the flood stretched right to it. The flood was perhaps ten meters deep, judging by the giant boulders that rolled downstream like Big Man’s bowling balls, smashing ice to shards and leaving steaming black polnyaps in their wake. The water in the open patches seemed to be moving at perhaps thirty kilometers an hour. So (punching figures into her wristpad) perhaps four and a half million cubic meters per hour. That was about a hundred Amazons out there, but running irregularly, freezing and bursting in a perpetual series of ice dams building and failing, whole steaming lakes leaping downhill over whatever channel or slope they found themselves on, stripping the land down to bedrock and then tearing the bedrock away… Lying on the floor of the rover, Ann could feel that assault in her cheekbones, vibrating the ground in a rapid pounding. Such tremors hadn’t been felt on Mars in millions of years, which explained something else that she had seen but not been able to comprehend; the northern wall of Ius was moving. The rock of the cliffs was flaking off and falling into the canyon, which shook the ground, and triggered more collapses, and giant waves that washed out into the flood, water pouring back upstream over the ice, the rock bursting apart in explosions of hydration, the frost steam pouring so thickly into the dust-choked air that she could see the northern wall only in snatches.

And without a doubt the southern wall would be collapsing in a similar way, although their view of this wall, which loomed over their road to the right, was foreshortened and cut off. But it had to be falling. And if it flaked off above them, then they were dead for sure. It was quite possible-very possible. Judging by her glimpses of the north wall, the chances might be as high as fifty percent. But then it was probably worse over there; the northern wall appeared to be undercut by the flood, while the south wall was removed from it by the bench they were driving over. So the southern cliffs should be a bit more stable-

But then something drew her eye forward, downstream from them. Up there the south wall was indeed collapsing, falling in great sheets of rock. The base of the cliff exploded in a cloud of dust that bloomed over the talus, and the upper sections of the cliff slid down into this new cloud of dust and disappeared. After a second the whole mass reappeared flying horizontally out of the cloud, a strange sight. The noise was painfully loud, even inside the car; then it was just a long, slow landslide, down into the flood, the rocks crushing the ice and blocking the flow beneath. An instant dam, cutting off much of the flow downcanyon; and so the banks of the flood began to rise. Ann watched the icy sheet of the shoreline below her rupture, and then it was chunks of ice, jostling in a sea of black smoking fizzing water, rising swiftly toward the rover. It would engulf them if the landslide dam lasted long enough. Ann peered at the long black spill of rock ahead of them; only a strip of it was still visible above the flood. But the slush beneath her continued to rise. It was a race of sorts. Big Man’s bathtub, draining while he poured new bucketfulls in. The speed of the lake’s ascent caused Ann to raise her estimate of the flow rate. She felt paralyzed, disconnected, in some curious sense serene; it was a matter of indifference to her whether or not the dam broke before the flood reached them. And in the overwhelming roar she felt no need to communicate with the others about this; it was impossible. She found that in a way she was cheering the flood on. It would serve them all right.

But then the landslide dam disappeared under the discolored slurry, and it all rolled off downstream in a stately collapse, the short-lived lake dropping as she watched, ice blocks on its surface clattering together, shrieking and booming as they collided and jumbled around and shot high into the air, all fantastically loud, every audible pitch roaring at once. It had to be well over a hundred decibels. She had her fingers in her ears, but couldn’t remember for how long. The car was bouncing up and down. She could see more landslides from the cliffs farther downstream, no doubt undercut by the sudden surge of the flood; and the tremors they caused were triggering further collapses, until it looked like the whole canyon would fill. It seemed impossible in all the noise and vibration that their little cars would survive. The travelers clutched their chair arms or lay there on the floor like Ann, isolated by the roar, their veins pumping with an awful mix of ice and adrenalin; even Ann, who did not care, found her breath short, her muscles tensed against the kinetic assault.

When they could hear each other’s shouts again, they asked Ann what had happened. Dully she stared out the window, ignoring them. Apparently they were going to survive, for the moment at least. The flood surface was now the most shattered chaotic terrain she had ever seen, the ice pulverized to a plain of wicked shards. The high point of the lake had climbed their bench until it had been only a hundred meters downslope from them; the re-exposed wet ground down there had turned from rusty black to dirty white in less than twenty seconds. Freezing time on Mars.

Sax had stayed in his seat through all that, absorbed by flickering on his screen. A lot of water would evaporate, or rather freeze and sublime, he muttered to no one as he worked. It was a heavily carbonated saline brine, but it would end up as dust-filled snow, falling somewhere else. The atmosphere might get hydrated enough so that it would snow several times, or even on a regular basis, in cycles of precipitation and sublimation. Thus the floodwater would get distributed pretty evenly planetwide, except perhaps at the highest altitudes. Albedo would rise dramatically. They would have to lower it, presumably by encouraging the snow algae that the Acheron group had created. (But there was no more Acheron, Ann said to him in her mind.) Black ice would melt by day, then freeze at night. Sublimate and precipitate. And thus they would have a waterscape: streams collecting, pooling, running downstream, freezing and expanding in cracks in the rock, subliming and snowing and melting and running again. A glaciated or muddy world, most of the time. But a waterscape nevertheless.

And every single feature of the primal Mars would melt away. Red Mars was gone.

Ann lay there on the floor by the window. Her tears poured out of her to join the flood; over the dam of her nose, downstream until her right cheek and ear and the whole side of her face was wet.

“This will complicate the process of getting downcanyon,” Michel said with a little Gallic smile, and from the next car Frank laughed. In fact it looked as if it would be impossible for them to proceed even five kilometers. Directly before them the canyon highway was buried under the great landslide, completely gone. The new spill of rock was shattered and unstable, sapped from below by the flood, pounded from above by subsequent mass wasting of the new slope.

For a long time the others debated even making a try. They had to speak loudly to be heard over the jet engine roar of the flood, which still swept past with no sign of a let-up. Nadia considered the slope suicidal, but Michel and Kasei were pretty sure they could find a way, and after a long day’s reconaissance on foot they managed to convince Nadia to agree to try it, and the rest were willing if Nadia was. And so the day after that, protected from surveillance by the general dust storm and the flood’s steam, they divided into the two cars and drove slowly out onto the slide.