"Ancillary wonder," Farley said mysteriously. "A result of the other wonders, if you follow Rimskaya and Takahashi's theories. But you're the space-time expert here."

"I'm not so sure," Patricia said distantly.

"If the corridor is a matrix of bent geodesics, a warped tube of space, what would you expect to find at its center?"

"I wondered about that yesterday afternoon." She paused as the cart neared the end of the staging area. "It's not going to work down the center. There's going to be a region where all the rules fail."

"Precisely."

"A singularity?"

"That's where we're going," Farley said.

The guard pulled the cart alongside an airlock mounted in the rock wall. Farley gripped a guide bar and helped Patricia out of her belt. The guard saluted and said he would wait for them.

They entered the airlock. Farley switched on a light and pulled down two rumply, one-size-fits-all pressure suits from a rack. "You can snug the arm and leg lengths a bit with these straps. Mobility and finesse aren't really needed here, just pressure and temperature and air. This isn't the most visited spot on the Stone."

The rear wall of the airlock was equipped with a broad-runged ladder, ascending to a wheel-opened hatch in the ceiling. Bits and pieces of equipment—some obviously long-abandoned—lay stocked in the corners and under the ladder.

"Just watch your step. Take everything slowly. There's no danger if you're careful. If anything happens to your suit—very unlikely—we can be back in the lock in less than two minutes."

Farley checked Patricia's suit seals and pressed a red button on a panel mounted near the ladder. The air was quietly pumped out of the chamber until Patricia could hear only her own breathing. Farley switched on their suit radios.

"Up the ladder," she said.

"I've never been in a spacesuit before," Patricia said, climbing the rungs after Farley.

"You didn't get spacesick, according to the OTV crew."

"Being weightless is fun."

"Hmm. Took me three days to get used to it."

Farley spun the wheel on the hatch and pushed it open. It glided slowly upward, then stopped until she ascended another rung and gave it a push. It swung out of the way. Compact floodlights had been installed in the bore hole, though the opening to the seventh chamber was only a dozen meters away, and the milky glow of the inner plasma tube spread faintly throughout.

Patricia turned to look south. The walls of the bore hole—rough and grooved with irregular lines—faded into inky blackness. At the end of the blackness was a circle of light the size of a BB held at arm's length. She looked up as best she could and saw a wide intrusion of dark rock in the asteroidal metal.

"Plasma tube begins anew in each chamber," Farley said. "It butts against the caps, sustained by a very weak bottle. The bottle also acts to keep the atmosphere in—otherwise, it would all have licked out through the bore holes. Leaked, I mean. Leak?"