The Ringworld Engineers

They came to a wall of falling water.

From a mile distant it appeared perfectly straight and infinitely long. The top of the waterfall was twenty miles above their heads. The base was hidden in fog. Water thundered in their ears until Chmeee had to turn off the microphones, and then they could hear it through the hull.

“It’s like the water condensers in the city,” the boy said. “This must be where my people learned how to make water condensers. Chmeee, did I tell you about water condensers?”

“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 altitude, 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-God 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.”

“Yah. 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 space 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 skyscraper. 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 god-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 pass a war fleet or a patch to cover the crater in Fist-of-God. 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 basement. I ca

“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 fu

“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.