The Qayaq airfield had to exist because of the Ashwall. West of here, all the way to 166 Thirty and beyond, the chain of volcanoes formerly known as the Kenai and Alaska Peninsulas and the Aleutian chain were in a nearly continual state of eruption. Any pilot meaning to fly north or south across the sixtieth parallel, in the zone bracketed by 166 Thirty on the west and the Rockies on the east, had to make allowances in the flight plan for the likelihood that their path would abruptly be barred by a plume of volcanic ash hurled into the stratosphere by any of a hundred active volcanoes lying upwind. Airplanes were expensive, even more so than they had been on Old Earth. They were too large to manufacture in the ring and transport down to the surface and so they, and other large productions such as arks and ships, had to be built in factories on the surface. Typically these lay on the outskirts of Cradle sockets. In any case, planes had to be babied, given that high-capacity turbofan engines were extraordinarily difficult to make. Every flight plan had to include a possible emergency landing on the artificial floe of Qayaq, which in turn had to be fully capable of hosting big airplanes. So what had been conceived as an emergency landing site had become something of a hub, where planes tended to land just because it was convenient and predictable. In any case it happened to be the final destination of the military flight on which the Seven had hitched a ride, so they had to get off here anyway.