"Yes. If the City Builders came this far, one wonders if they found the way inside. Do your tales tell anything of a hollow land?"

"No."

Louis said, "Their magicians are all built like Pak protectors."

The boy asked, "Luweewu, this great waterfall-why is there so much of it?"

"It must run all the way round the top of the Map. It takes out the water vapor. The top of the Map has to be kept dry," Louis said. "Hindmost, are you listening?"

"Yes. Your orders?"

"We'll circle with the lander, using deep-radar and the other instruments. Maybe we'll find a door under the waterfall. We'll use Needle to explore the top. How's our fuel supply?"

"Adequate, given that we won't be going home."

"Good. We'll dismount the probe and set it following Needle at ... ten miles and ground-hugging alt.i.tude, I think. Keep the stepping-disc links and the microphones open. Chmeee, do you want to fly the lander?"

The kzin said, "Aye, aye."

"Okay. Come on, Kawa."

"I'd like to stay here," the boy said.

"Harkabeeparolyn would kill me. Come on."

Needle rose twenty miles, and red Mars stretched before them.

Kawaresksenjajok said, "It looks awful."

Louis ignored that. "At least we know we're looking for something big. Picture a blowout patch big enough to plug Fist-of-G.o.d Mountain. We want a hatch big enough for that patch plus the vehicle to lift it. Where would you put it on the Map of Mars? Hindmost?"

"Under the waterfall," the Hindmost said. "Who would see? The ocean is empty. The failing water would hide all."

"Yeah. Makes sense. But Chmeee's searching that. Where else?"

"I must hide the lines of a gigantic hatch in a martian landscape? Perhaps an irregular shape, with hinges in a long, straight canyon. Perhaps I would put it beneath the ice, melting and refreezing the north pole to conceal my comings and goings."

"Is there a canyon like that?"

"Yes. I did my homework. Louis, the poles are the best gamble. Martians never went near the poles. Water killed them."

The Map was a polar projection; the south pole was spread out around the rim. "Okay. Take us to the north pole. If we don't find anything, we'll spiral out from there. Stay high and keep all instruments going. We don't care too much if something fires on Needle. Chmeee, are you listening?"

"I hear."

"Tell us everything. Chances are you'll find what were after. Don't try to do anything about it." Would he obey? "We don't invade in the lander. We're burglars. We'd rather be shot at in a General Products hull."

Deep-radar stopped at the scrith floor. Above the scrith the mountains and valleys showed translucent. There were seas of marsdust fine enough to flow like oil. Under the dust were cities of a sort: stone buildings denser than the dust, with carved walls and rounded corners and a good many openings. The City Builders stared, and so did Louis Wu. Martians had been extinct in human s.p.a.ce for hundreds of years.

The air was clear as vacuum. Off to starboard, well beyond the horizon, was a mountain taller than any on Earth. Mons Olympus, of course. And a splinter of white floated above the crater.

Needle fell, and pulled out of the fall just above the crescent dunes. The structure was still visible, floating fifty to sixty yards above the peak; and Needle must have been quite visible to its occupants.

"Chmeee?"

"Listening."

Louis fought a tendency to whisper. "We've found a floating skysc.r.a.per. Maybe thirty stories tall, with bay windows and a landing ledge for cars. Built like a double cone. It looks very much like the building we took over on our first trip, the good ship Improbable."

"Identical?"

"Not quite, but close. And it's floating above the highest mountain on Mars, just like a G.o.d-tanjed signpost."

"It does sound like a signal meant for us. Shall I flick through?"

"Not yet. Have you found anything?"

"I believe I've traced the lines of a tremendous hatch inside the waterfall. It would pa.s.s a war fleet or a patch to cover the crater in Fist-of-G.o.d. There may be signals to open it. I haven't tried."

"Don't. Stand by. Hindmost?"

"I have radiation and deep-radar scans. The building is radiating little energy. Magnetic levitation does not require large amounts of power."

"What's inside?"

"Here." The Hindmost gave them a view. By deep radar the structure showed translucent gray. It appeared to be a floating building modified for travel, with fuel tanks and an air-breathing motor built into the fifteenth floor. The puppeteer said, "Solid construction: walls of concrete or something equally dense. No vehicles in the carport. Those are telescopes or other sensor devices in the tower and the bas.e.m.e.nt. I cannot tell if the structure is occupied."

"That's the problem, all right. I want to outline a strategy. You tell me how it sounds. One: we go as fast as possible to just above the peak."

"Making perfect targets of ourselves."

"We're targets now."

"Not from weapons inside Mons Olympus."

"What the futz, we're wearing a General Products hull. If nothing fires on us, we go to step two: we deep-radar the crater. It we find anything but a solid scrith floor we go to step three: vaporize that building. Can we do that? Fast?"

"Yes. We don't have power storage to do it twice. What is step four?"

"Anything to get us inside quick. Chmeee stands by to rescue us any way he can. Now tell me whether you're going to freeze up halfway through this procedure."

"I wouldn't dare."

"Wait a bit." It came to Louis that their native guests were scared spitless. To Harkabeeparolyn he said, "If there is a place in the world where the world can be saved, that place is below us. We think we've found the door. Someone else has found it too. We don't know anything about him, or them. Understand?"

The woman said, "I'm frightened."

"So am I. Can you keep the boy calm?"

"Can you keep me calm?" She laughed raggedly. "I'll try."

"Hindmost. Go."

Needle leapt into the sky at twenty gravities, and rolled, and stopped upside down, almost alongside the floating building. Louis's belly rolled too. Both City Builders shrieked. Kawaresksenjajok had a death grip on his arm.

Eyesight showed the crater plugged by old lava. Louis watched the deep-radar image.

It was there! A hole in the scrith, an inverted funnel leading up (down!) through the crater in Mons Olympus. It was far too small to pa.s.s Ringworld repair equipment. This was a mere escape hatch, but it was roomy enough for Needle.

"Fire," Louis said.

The Hindmost had last used this beam as a spotlight. At close range it was devastating. The floating building became a streamer of incandescence with a cometlike head of boiling concrete. Then it was only dust cloud.

Louis said, "Dive."

"Louis?"

"We're a target here. We don't have time. Dive. Twenty gravities. We'll make our own door."

The ocher landscape was a roof over their heads. Deep-radar showed a hole in the scrith, dropping to engulf them. But every other sense showed the solid lava crater in Mons Olympus descending at terrible speed to smash them.

Kawaresksenjajok's nails in Louis's arm were drawing blood. Harkabeeparolyn seemed frozen. Louis braced for the impact.

Darkness.

There was formless, milky light from the deep-radar screen. Something else was glowing somewhere: green and red and orange stars. Those were dials on the flight deck.

"Hindmost!"

No answer.

"Hindmost, give us some light! Use the spotlight! Let us see what's threatening us!"

"What happened?" Harkabeeparolyn asked plaintively. Louis's eyes were adjusting; he could see her sitting on the floor, hugging her knees.

Cabin lights came on. The Hindmost turned from the controls. He looked shrunken: half curled up already. "I can't do this any more, Louis."

"We can't use the controls. You know that. Give us a spotlight so we can see out."

The puppeteer touched controls. A white diffused light bathed the hull in front of the flight deck.

"We are embedded in something." One head glanced down; the other said, "Lava. The outer hull is at seven hundred degrees. Lava was poured over us while we were in stasis and is now cooled."

"Sounds like someone was ready for us. Are we still upside down?"

"Yes."

"So we can't accelerate up. Just down."

"Yes."

"Want to try it?"

"What are you asking? I want to start over from just before you burned out the hyperdrive motor-"

"Come on, now."

"-or from just before I decided to kidnap a man and a kzin. That was probably a mistake."

"We're wasting time."

"There is no place to radiate Needle's excess heat. Using the thrusters would bring us an hour or two closer to the moment when we must go into stasis and await developments."

"Hold off for a while, then. What are you getting from deep-radar?"

"Igneous rock in all directions, cracked with cooling. Let me expand the field ... Louis? Scrith floor some six miles below us, below Needle's roof. A much thinner scrith ceiling fourteen miles above."

Louis was beginning to panic. "Chmeee, are you getting all this?"

He was answered in unexpected fashion.

He heard a howl of inhuman pain and rage as Chmeee burst from the stepping disc, running full out with his arms across his eyes. Harkabeeparolyn dove out of his path. The water bed caught the kzin across the knees and he rolled across the bed and onto the floor.

Louis had leaped for the shower. He flipped it on full blast, jumped the water bed, put his shoulder into Chmeee's armpit, and heaved. Chmeee's flesh was hot beneath the fur.

The kzin stood and followed the pull into the stream of cold water. He moved about, getting water over every part of himself; then he huddled with his face in the stream. Presently he said, "How did you know?"

"You'll smell it in a minute," Louis said. "Scorched fur. What happened?"

"Suddenly I was burning. A dozen red lights glowed on the board. I leaped for the stepping disc. The lander is still on autopilot, if it isn't destroyed."

"We may have to find out. Needle's embedded in lava. Hindmost?" Louis turned toward the flight deck.

The puppeteer was curled up with his heads beneath his belly.

One shock too many. It was easy to see why. A screen on the flight deck showed a half-familiar face.

The same face, enlarged, was looking out of the rectangle that had been a deep-radar projection. A mask of a face, like a human face molded out of old leather, but not quite. It was hairless. The jaws were hard, toothless crescents. From deep under a ridge of brow, the eyes looked speculatively out at Louis Wu.

Chapter 30 -.

Wheels Within Wheels "It appears you've lost your pilot," the leathery-faced intruder told them. It floated outside the hull: the distorted head and melon-sized shoulders of a protector, a ghost within the black rock that enclosed them.