This, he concluded, was the caldera of Pavonis Mons. They had landed at Sheffield, so really there could be no doubt about it. Therefore it was some sixty kilometers across the circle to that observatory, as Art recalled from his video documentaries, and five kilometers to the floor. And all of it completely empty, rocky, untouched, primordial— the volcanic rock as bare as if cooled the week before— nothing at all of humanity in it— no sign of terraforming. It must have looked exactly like this to John Boone, a half century before. And so . . . alien. And big. Art had looked into the calderas of Etna and Vesuvius, while on vacation from Tehran, and those two craters were big by Terran standards, but you could have lost a thousand of them in this, this thing, this hole. . . .

He closed the drapes and got slowly dressed, his mouth imitating the shape of the unearthly caldera.

• • •

A friendly Praxis guide named Adrienne, tall enough to be a Martian native but possessing a strong Australian accent, collected him and took him and half a dozen other new arrivals on a tour of the town. Their rooms turned out to be on the city’s lowest level, though it wouldn’t be lowest for long; Sheffield was in the process of burrowing downward these days, to give as many rooms as possible the view onto the caldera that had so disconcerted Art.

An elevator took them up nearly fifty stories, and let them out in the lobby of a shiny new office building. They walked out its big revolving doors and emerged on a wide grassy boulevard, and walked down it past squat buildings faced with polished stone and big windows, separated by narrow grassy side streets, and a great number of construction sites, as many buildings were still in various stages of completion. It was going to be a handsome town, the buildings mostly three and four stories tall, getting taller as they moved south, away from the caldera rim. The green streets were crowded with people, and the occasional small tram running on narrow tracks set in the grass; there was a general air of bustle and excitement, caused no doubt by the arrival of the new elevator. A boom town.

The first place Adrienne took them was across a boulevard to the caldera rim. She led the seven newcomers out into a thin curving park, to the nearly invisible tenting that encased the town. The transparent fabrics were held in place by equally transparent geodesic struts, anchored in a chest-high perimeter wall. “The tenting has to be stronger than usual up here on Pavonis,” Adrienne told them, “because the atmosphere outside is still extremely thin. It’ll always be thinner than the lowlands, by a factor of ten.”