"Have you ever considered how much dirt they had to bring into the corridor?" she asked, standing up. She had taken only a few bites from her sandwich. She hadn't been hungry for two days. "If the dirt is about a quarter kilometer deep—"

"So we estimate from sounding," Takahashi said.

"And let's say the corridor is a billion kilometers long. . .”

"Why that figure?"

"Just a guess," she said. "That makes about forty billion kilometers of dirt."

"If we broke the Earth down and paved the corridor with it—crust, magma and core—we could cover about thirty billion kilometers." Takahashi poked his finger into a sandy patch.

"What if they have mountains farther on? Even more dirt and rock required, then."

"That's possible," Takahashi said, "But the big question is, Where did they get it all? And don't forget the air. It's about twenty kilometers deep, so that would make. . . one point six trillion cubic kilometers of air, at just over a gram a liter—"

"You've worked all this out before."

"Of course. Many times. Rimskaya started it, and the statistics people carried on. I kibitzed. So many questions about logistics and design. How does the air get renewed in the corridor? The Stone's regeneration ponds couldn't possibly keep up with it, not if there's any sizable population of animals farther on. So maybe there's just enough air to last a few thousand years."

"That doesn't seem right," Patricia said. "Whoever—or whatever—set this up, designed it for eternity. Don't you get that feeling?"

"Sometimes. Doesn't mean it's a valid assumption."

"Still, there must be some kind of corridor maintenance system."

Takahashi nodded. "Rimskaya theorized there would be openings in the corridor even before we discovered the wells."

Carrolson joined them. "Ever notice what the corridor smells like?" she asked.

Patricia and Takahashi shook their heads.

"Smells just like before a storm. All the time. But the ozone levels aren't very high. Another mystery."

Patricia sniffed the air. It smelled fresh, but not like a brewing storm.

"I was raised in storm country," Carrolson said defensively. "That's the smell, all right."

Back in the truck, continuing the journey, Patricia spent much of the time doing problems on the processor, figuring volumes and masses and putting them all into a small table.

An hour later, Takahashi pointed out the first circuit, four wells at the quadrilateral points of a ring. Each well sat in the middle of a dimple about half a kilometer in diameter and twenty meters deep. In the center of the dimple was an inverted bronze-colored dish fifteen meters wide, suspended eight meters above the bowl. The dish hovered in empty air, unsupported.

The truck slowed near the rim of the dimple. At Takahashi's request, Lake drove them around the well before stopping. They climbed down and approached the rim.

"We've made about twenty trips to this circuit," Takahashi said. "Beaten a path, almost."