He turned, and Teela was in the airlock. She was not wearing a pressure suit. The inner door was just closing. He bellowed, "Teela, you silly leucoto, come out of there!"

Too late. She couldn't possibly have heard him through the closed hermetic seal. Louis sprang to the lockers.

The air samplers on the Liar's wing had been vaporized with the rest of the Liar's external sensors. He would have to go out in a pressure suit and use the chest sensors to find out if the Ringworld's air could be breathed safely.

Unless Teela collapsed and died before he could get out. Then he would know.

The outer door was opening.

Automatically the internal gravity went off in the airlock. Teela Brown dropped headfirst through the open door, clutched frantically for a door jamb, had it for just long enough to change her angle of fall. She landed on her tail instead of her skull.

Lows climbed into his pressure suit, zipped up the chest, donned the helmet and closed the clamps. Outside and overhead, Teela was on her feet, rubbing herself where she had landed. She hadn't stopped breathing, thank Finagle for his forebearance.

Louis entered the lock. No point in checking his suit's air. He'd only be in the suit long enough for the instruments to tell him if he could breath outside air.

He remembered the tilt of the ship in time to grab at the jamb as the airlock opened. As the cabin gravity went off Louis swung around, hung by his hands for an instant, and dropped.

His feet shot out from under him the moment they touched ground. He landed hard on his gluteus maximi.

The flat, grayish, translucent material beneath the ship was terribly slippery. Louis tried once to stand, then gave it up. Sitting, he examined the dials on his chest.

His helmet spoke to him in Speaker's furry voice.

"Louis."

"Yeah."

"Is the air breathable?"

"Yeah. Thin, though. Say a mile above sea level, Earth standard."

"Shall we come out?"

"Sure, but bring a line into the lock and tie it to something. Otherwise we'll never get back up. Watch out when you get down. The surface is almost frictionless."

Teela was having no trouble with the slippery surface. She stood awkwardly, with her arms folded, waiting for Louis to quit fooling around and take off his helmet.

He did. "I have something to tell you," he said. And he spoke rudely to her.

He spoke of the uncertainties in spectroanalysis of an atmosphere from two light years away. He spoke of subtle poisoin metal compounds, and strange dusts, organic wastes and catalysts, which can poison an otherwise breathable atmosphere, and which can only be detected from on actual air sample. He spoke of criminal carelessness and culpable stupidity; he spoke of the unwisdom in volunteering one's services as a guinea pig. He said it all before the aliens could leave the airlock.

Speaker came down hand over hand, landed on his feet and moved a few steps away, cat-careful, balanced like a dancer. Neesus came down gripping the rope with alternate sets of teeth. He landed in tripod position.

If either of them noticed that Teela was upset, they gave no sign. They stood below the tilted hull of the Liar, looking about them.

They were in an enormous, shallow gully. Its floor was translucent gray and perfectly flat and smooth, like a vast glass tabletop. Its borders, a hundred yards from the ship in either direction, were gentle slopes of black lava. The lava seemed to ripple and flow before Louis's eyes. It must be still hot, he decided, from the impact of the Liar's landing. The shallow lava walls stretched away behind the ship, away and away, perfectly straight, until they dwindled to a vanishing point.

Louis tried to stand up. Of the four of them he was the only one having trouble with his balance. He reached his foot, then stood precariously balanced, unable to move.

Speaker-To-Animals unsheathed his flashlight-laser and fired at a point near his feet. They watched the point of green light … in silence. There was no crackle of solid material exploding into vapor. No steam or smoke formed where the beam struck. When Speaker released the trigger button, the light was gone instantly; the spot was not glowing, nor was it marked in any way.

Speaker delivered the verdict. "We are in a furrow plowed by our own landing. The ring foundation material must have ultimately stopped our fall. Nessus, what can you tell us about it?"

"This is something new," the puppeteer answered. "It seems to retain no heat. Yet it is not a variant on the General Products hall, nor on the Slaver stasis field."

"We'll need protection to climb the walls," said Louis. He wasn't particularly interested in the ring foundation material. Not then. "You'd better stay here, all of you, while I climb up."

After all, he was the only one wearing a heat-insulated pressure suit.

"I'll come along," said Teela. Moving without effort, she came up under his arm. He leaned heavily on her, stumbling but not falling, as they moved toward the black lava slope.

The lava was good footing, though steep. "Thanks," he said, and he started up. A moment later he realized that Teela was following him. He said nothing. The faster she learned to look before she leapt, the longer she'd live.

They were a dozen yards up the slope when Teela yelled and began dancing. Kicking high, she turned and pelted downslope. She slid like an ice skater when she hit the ring floor. Sliding, gliding, she turned with her hands on her hips and glared upward, baffled and injured and angry.

It could have been worse, Louis told himself. She could have slipped and fallen and burned her bare hands — and he'd still have been right. He continued to climb, repressing ugly pangs of guilt.

The bank of lava was approximately forty feet high. At the top it gave way to clean white sand.

They had landed in a desert. Searching the near distance with his eyes, Louis could find no sign of vegetation-green or water-blue. That was a piece of luck. The Liar could as easily have plowed through a city.

Or through several cities! The Liar had plowed quite a furrow …

It stretched long miles across the white sand. In the distance, beyond where that gouge ended, another began. The ship had bounced, not once, but many times. The gouge of the Liar's landing went on and on, narrowing to no more than a dotted line, a trace … Louis let his eyes follow that trace, and he found himself looking into infinity.

The Ringworld had no horizon. There was no line where the land curved away from the sky. Rather, earth and sky seemed to merge in a region where details the size of continents would have been mere points, where all colors blended gradually into the blue of sky. The vanishing point held his eyes fixed. When he blinked, as he finally did, it was with deliberate effort.

Like the void mist of Mount Lookitthat, seen decades ago and light-centuries away … like the undistorted deeps of space, as seen by a Belt miner in a singleship … the Ringworld's horizon could grip the eye and the mind of a man before he was aware of the danger.

Louis turned to face into the gully. He shouted, "The world is flat!"

They looked up at him.

"We ripped quite a line coming down. I can't see that there's anything living around here, so we were lucky. Where we hit, the earth splashed; I can see a scattering of small craters, secondary meteorites, back along the way we came.

He turned. "In the other direction …" and he stopped.

"Louis?"

"That's the biggest tanj mountain I ever saw in my life."

"Louis!"

He had spoken too softly. "A mountain!" he bellowed. "Wait'll you see it! The Ringworld engineers must have wanted to put one big mountain in the world, one mountain too big to use. Too big to grow coffee on, or trees, too big even for skiing. It's magnificent!"