For the work of recollection to best succeed, it was becoming clear from the experiments that context was an important component. Lists memorized underwater in diving suits could be recalled much better when the subjects returned to the seafloor than when they tried to remember them on land. Subjects hypnotically induced to feel happy or sad during memorization of a list were better at remembering the list when again hypnotized to feel happy or sad. Congruence of items in the lists helped, as did returning to rooms of the same size or color when remembering them. These were of course all very crude experiments, but the link between context and power of recollection was demonstrated by them strongly enough to cause Sax to think hard about where he might want to try the treatment when they finalized it; where, and with whom.

For the final work on the treatment Sax called up Bao Shuyo and asked her to come join them in Acheron for some consulations. Again, her work was much more theoretical, and very much more fine-grained, but after her work with the fusion group in Da Vinci, he had a healthy respect for her ability to help in any problem that involved quantum gravity and the ultramicrostructure of matter. Just to have her run through what they had done and comment on it would be valuable, he was sure.

Unfortunately, Bao’s obligations in Da Vinci were heavy, as they had been ever since her much-heralded return from Dorsa Brevia. Sax was put in the unusual position of manipulating his home labs in order to extricate– one of their best theorists, but he did it without compunction, getting Bela’s help to put the arm on the current administration, to twist their arms as hard as ever he could. “Ka, Sax,” Bela exclaimed during one call, “I never would have guessed that you would turn out to be such a fierce headhunter.” “It’s my own head I’m hunting,” Sax replied.

Usually tracking someone downwas as simple as contacting their wristpad, and looking to see where the person was. Ann’s wristpad, however, had been left on the rim of the Olympus Mons caldera, at the descent station near the festival grounds at Crater Zp. This struck Sax as peculiar, since they had worn wristpads of some kind or another since the very beginning in Underbill, Ann as much as anyone, as he recalled. Hadn’t she? He called Peter to ask, but Peter did not know, of course, having been born well after the Underbill years. In any case, to go without a wristpad now was to borrow a behavior from the neoprimitive nomads wandering the canyonlands and the North Sea coast — not a lifestyle he would have expected Ann to take any interest in. One couldn’t live in anything like the Paleolithic style up on Olympus Mons, indeed it required the kind of continuous technological support that was no longer necessary in most places, with wristpads an integral part of it. Perhaps she only wanted to get away. Peter didn’t know.

But he did know how to contact her: “You have to go in and find her.”

At Sax’s expression he laughed. “It isn’t so bad. There’s only a couple hundred people in the caldera, and when they’re not staying in one of their huts, they’re on the cliff walls.”

“She’s become a climber?”

“Yes.”

“She climbs for — for recreation?”

“She climbs. Don’t ask me why.”

“So I just go look at all the cliffs?”

“That’s how I had to do it when Marion died.”

The summit of Olympus Mons had for the most part been left alone. Oh there were a few low boulder hermitages on rim overlooks, and a piste had been built on the northeast lava flow that broke the escarpment ring surrounding the volcano, for easy access to the festival complex at Crater Zp; but other than that, there was nothing to show what had happened to the rest of Mars, which from the rim of the caldera was entirely invisible, under the horizon of the encircling escarpment. From its rim Olympus Mons appeared to be the world entire. The local Reds had decided against putting a protective molecular dome over the caldera, something they had done over Arsia Mons; so no doubt there were bacteria, and perhaps some lichens that had blown over on winds and floated down into the caldera and survived; but at pressures little higher than the original ten millibars, they were not going to flourish. Probably the survivors were mostly endochasmoliths, so there would be no sign of them. It was a lucky thing for the Red project that Mars’s stupendous vertical scale kept air pressures so low on the big volcanoes; a free and effective sterilization technique.

Sax took the train up to Zp, and then a car on up to the rim, a taxi van driven by the Reds who controlled access into the caldera. The car came to the edge of the rim, and Sax looked down.

The caldera was multiringed, and big: ninety kilometers by sixty, about the same size as Luxembourg, Sax recalled hearing. The main central circle, by far the largest, was marred by overlapping smaller circles to the northeast, center, and south. The southernmost circle cut in half a slightly older, higher circle to the southeast; the meeting of these three arcuate walls was considered one of the finest climbing areas on the planet, Sax was told, with the greatest height anywhere in the caldera, a drop from 26 kilometers above the datum (they used the old term rather than sea level) down to 22.5 kilometers on the southernmost crater floor. A ten-thousand-foot cliff, the young Coloradoan in Sax mused.

The floor of the main caldera was marked by a great number of curving fault patterns, concentric with the caldera walls: arcing ridges and canyons, across which ran some straighter escarpments. These features were all explicable, they had been caused by recurrent caldera collapses following the sideslope drainage of magma from the main chamber under the volcano; but as he looked down from their perch on the rim, it seemed to Sax a mysterious mountain — a world of its own — nothing visible but the vast embayed rim, and the five thousand square kilometers of the caldera. Ring on ring of high curved walls and flat round floors, under a black starry sky. Nowhere were the encircling cliffs less than a thousand meters tall. As a rule they were not completely vertical; their average slope appeared to be just steeper than forty-five degrees. But there were steeper sections all over the place. No doubt the climbers flocked to the very steepest sections, given the nature of their interest. There looked to be some very vertical faces out there, even an overhang or two, as right under them, over the confluence of the three walls.

“I’m looking for Ann Clayborne,” Sax said to the drivers, who were rapt with the view. “Do you know where I could find her?”

“You don’t know where she is?” one asked.

“I’ve heard she’s climbing in the Olympus caldera.”

“Does she know you’re looking for her?”

“No. She’s not answering her calls.”

“Does she know you?”

“Oh yes. We’re old — friends.”

“And who are you?”

“Sax Russell.”

They stared at him. One said, “Old friends, eh?”

Her companion elbowed her.

They called the spot they were at Three Walls, sensibly enough. Directly under their car, on a little slump terrace, there was an elevator station. Sax peered at it through binoculars: outer-lock doors, reinforced roofing — it could have been a structure from the early years. The elevator was the only way down into this part of the caldera, if you did not care to rappel.

“Ann resupplies at Marion Station,” the elbower finally said, shocking her codriver. “See it, there? That square dot, where the lava channels from the main floor cut down into South Circle.”

This was on the opposite rim of the southernmost circle, which Sax’s map named “6.” Sax had trouble making out any square dot, even with the binocular’s magnification. But then he saw it — a tiny block just a bit too regular to be natural, although it had been painted the rusty gray of the local basalt. “I see it. How do I get there?”