They replaced various instruments in ports along the floor of the aircraft and installed new sensors in ports so far unoccupied. Lanier stared overhead at the corridor floor, fascinated by the procession of lights. Even with binoculars, he couldn't make the lights resolve into anything but bright spots, contrasting against the black of the lanes.

Something large and gray covered his field of view in the binoculars and he pulled them down. A disk at least half a kilometer wide floated slowly above the lanes, moving south. Another disk followed a similar course twenty or thirty degrees to the west.

"Absolutely no coherent radio signals," Heineman said. "Waste microwaves and heat and a little X- and gamma-ray activity and that's it. Radar—the repeater back here shows something substantial about a quarter of a million kilometers ahead—surface area of at least fifteen square kilometers, right on the axis—dead center."

"I see it," Lanier said, looking at the primary display. "Objects moving around it, and all along the wall of the corridor."

"Don't ask me what they are," Heineman said, peering through the windscreen at the gray disks. He squinted in puzzled anxiety. "And don't ask me how long we're going to stay up here unmolested."

"At least we're small," Farley said. "Maybe they won't notice us."

"That big thing up ahead, whatever it is, will notice us," Heineman said. "Ten to one it's riding the singularity, too."

Five hundred kilometers past the wall, four large brick-red twisted pyramids rose above the tangle of lanes. From their spacing—equidistant around the circumference, at the quadratic points—Lanier surmised they were built over wells. From this distance, they appeared the size of a commemorative postage stamp held at arm's length—which made them perhaps two kilometers on a side, and a kilometer high. Kilometer-wide clear lanes extended straight north from each structure, for as far as he could see.

"I think we're in over our head," Lanier murmured.

Farley put her hand on his shoulder and pulled herself into the co-pilot's seat. "We've been over our heads for years, haven't we?"

"I'd always assumed the corridor was empty—I don't know why. Perhaps because I couldn't have imagined this."

Heineman floated between them and gripped a bar on the instrument panel to steady himself while he programmed a flight plan. "We're going to accelerate to ten thousand klicks an hour, get as close as we can to that big object on the singularity—slowing down on the approach, so they won't think we' re going to ram them—then reverse and hightail for home. That is, of course, if you approve." He raised an eyebrow in Lanier's direction.

Lanier weighed the risks and realized he had no idea what they were.