Sep was associated with Praxis, apparently, and each tent or covered canyon was run by a separate organization. They pooled information and shared roving consultants and construction teams. Since they deemed themselves necessary services, they ran on a cooperative basis— on the Mondragon plan, one said, nonprofit version— though they made sure to provide their members with very nice living situations and lots of free time. “They think they deserve it, too. Because when something goes wrong they have to act fast or else.” Many of the covered canyons had had close calls, sometimes the result of meteor strike or other drama, other times more ordinary mechanical failures. The usual format for covered canyons had the physical plant consolidated at the higher end of the canyon, and this plant sucked in the appropriate amounts of nitrogen, oxygen and trace gases from the surface winds. The proportions of gases and the pressure range they were kept at varied from mesocosm to mesocosm, but they averaged around five hundred millibars, which gave some lift to the tent roofs, and was pretty much the norm for indoor spaces on Mars, in a kind of invocation of the eventual goal for the surface at the datum. On sunny days, however, the expansion of air inside the tents was very significant, and the standard procedures for dealing with it included simply releasing air back into the atmosphere, or else saving it by compressing it into huge container chambers hollowed out of the canyon cliffs. “So one time I was in Dao Vallis,” one of the techs said, “and the excess air chamber blew up, shattering the plateau and causing a big landslide that fell down onto Reullgate and tore open the tent roof. Pressures dropped to the local ambient, which was about two hundred and sixty, and everything started to freeze, and they had the old emergency bulkheads,” which were clear curtains only a few molecules thick but very strong, as Sax recalled, “and when they deployed automatically around the break, this one woman got pinned to the ground by the supersticky at the bottom of the bulkhead, with her head on the wrong side! We ran over to her and did some quick cut and paste and got her loose, but she almost died.”

Sax shivered, thinking of his own recent brush with cold; and 260 millibars was the pressure one would find on the peak of Everest. The others were already talking about other famous blowouts, including the time Hiranyagarba’s dome had fallen in its entirety under an ice rain, despite which no one had died.

Then they were descending over the great cratered high plain of Xanthe, coming down on the Da Vinci crater floor’s big sandy runway, which they had just started using during the revolution. The whole community had been preparing for years for the day when stealthing would become unnecessary, and now a big curve of copper-mirrored windows had been installed in the arc of the southern crater rim. There was a layer of snow in the bottom of the crater, which the central knob broke out of quite dramatically. It was possible they could arrange for a lake in the crater floor, with a central knob island, which would have as its horizon the circling cliffy hills of the crater rim. A circular canal could be built just under the rim cliffs, with radial canals connecting it to the inner lake; the resulting alternation of circular water and land would resemble Plato’s description of Atlantis. In this configuration Da Vinci could support, in near self-sufficiency, some twenty or thirty thousand people, Sax guessed; and there were scores of craters like Da Vinci. A commune of communes, each crater a city-state of sorts, its polis fully capable of supporting itself, of deciding what kind of culture it might have; and then with a vote in a global council of some kind. . . . No regional association larger than the level of the town, except for arrangements of local interchange . . . might it work?