The Hindmost called.

Today his mane was a swirling rainbow, dyed in streamers of primary colors. Behind him puppeteers flickered along lines of stepping discs, cl.u.s.tered at shop windows, brushed against each other without apology or resentment, all in a murmur of music with flutes and clarinets predominating: puppeteer language. The Hindmost asked, "What have you learned?"

"Little," said Chmeee. "We have wasted time. There was certainly a great solar flare seventeen falans ago-about three and a half years-but we guessed that much. The shadow squares closed to protect the surface. Their guidance system must operate independently of the Ringworld's."

"We could guess that too. No more?"

"Louis's hypothetical Repair Center is certainly inactive. This swamp below us was not designed. I imagine a major river silted up to block the outflow of a sea. We find a variety of hominids, some intelligent, some not. Of those who built the Ringworld we find no trace, unless they were Halrloprillalar's ancestors. I am inclined to think they were."

Louis opened his mouth ... and glanced down at a threshold pain in his leg. He found four kzinti claws just resting on his thigh. He shut his mouth. Chmeee continued, "We have not met any of Halrloprillalar's species. Perhaps they were never a dense population. We hear rumors of another race, the Machine People, who may rise to replace them. We go to seek them."

"The Repair Center is inactive, yes," the Hindmost said briskly. "I have learned much. I have put a probe to work-"

"You have two probes," Chmeee said. "Use both."

"I hold one in reserve, to refuel Needle. With the other I have learned the secret of the spill mountains. See-"

The far right screen showed a probe's-eye view. It raced along the rim wall; pa.s.sed something, too quick for detail; slowed, turned, moved back.

"Louis advised me to explore the rim wall. The probe had barely started its deceleration routine when it found this. I thought it worth investigating!"

There was a swelling on the rim wall-a tube hooked over the lip. It was molded, flattened against the rim wall, and was made of the same translucent gray scrith. The probe eased toward it until the camera was looking up into a pipe a quarter-mile across.

"Much of the Ringworld's design shows a brute-force approach," the Hindmost was saying. And the probe moved alongside the pipe, over the lip, and down the outer face of the rim wall to where the pipe disappeared into the foamed material that formed a meteor shield for the Ringworld's underside.

"I see," Louis said. "And it wasn't working?"

"No. I tried to trace the pipe and had some success."

The scene jumped. Now it showed dark racing motion as the probe cruised a good distance outward from the Ringworld. Inverted landscape pa.s.sed above, seen by infrared light. The probe slowed, stopped, moved upward.

If a meteor struck the Ringworld, it had to fall first from interstellar s.p.a.ce; and it struck with that velocity plus the Ringworld's own seven hundred and seventy miles per second. A meteor had struck here. The plasma cloud had drawn a savage gouge across hundreds of miles of sea bottom, vaporizing the protective foam. There in the gouge was a length of pipe a few hundred feet in diameter. It led up into the sea bottom.

"A recycling system," Louis murmured.

The puppeteer said, "Without some counterbalance to erosion, the Ringworld's topsoil would all be in the sea bottoms in a few thousand years. I expect the pipes ran from the sea bottoms along the underside and up over the rim wall. They deposit sea-bottom sludge on the spill mountains. Much of the water would boil away in the near vacuum at the peak, thirty miles high. The mountain gradually collapses under its own weight. Material moves from the rim walls inward, carried by winds and rivers."

Chmeee said, "Mere supposition, but plausible. Hindmost, where is your probe now?"

"I intend to bring it out from under the Ringworld and reinsert it into the rim transport system."

"Do that. Does the probe have deep-radar?"

"Yes, but the range is short."

"Deep-radar the spill mountains. The spill mountains are ... perhaps twenty to thirty thousand miles apart? Thus we may find on the order of fifty thousand spill mountains along both rim walls. A handful of those would make a fine hiding place for the Repair Center."

"But why should the Repair Center be hidden?"

Chmeee made a rude noise. "What if the subject races should revolt? What of an invasion? Of course the Repair Center is hidden, and fortified too. Search every spill mountain."

"Very well. I will scan the starboard rim wall in one Ringworld rotation."

"Scan the other rim afterward."

Louis said, "Keep the cameras going too. We're still looking for att.i.tude jets ... though I'm starting to think they had something else going."

The Hindmost clicked off. Louis turned to the window. It had been tickling at his attention all along: a pale thread that curved along the edge of the swamp, straighter than a river. Now he pointed out the barely visible pair of dots moving along its length. "I think we need a closer look at that. Why don't you take us down?"

It was a road. From a hundred feet up it was rough-surfaced, stony stuff: white stone poured in a stream. Louis said, "The Machine People, I presume. Shall we track those vehicles?"

"Let us wait until we are closer to the floating city."

Giving up a present opportunity seemed silly, but Louis was afraid to object. The kzin's tension was thick enough to smell.

The road avoided the low, wet areas. It seemed in good repair. Chmeee followed it at low speed, a hundred feet up.

Once they pa.s.sed a handful of buildings, the biggest of which seemed to be a chemical plant. Several times they watched boxy vehicles pa.s.s below them. They were seen only once. A box stopped suddenly, and humanoid shapes spilled out, ran in circles, then produced sticks which they pointed at the lander. A moment later they were out of sight.

There were great pale shapes in the wet jungle. They couldn't be glacier-scoured boulders; not here. Louis wondered if they might be tremendous fungi. He stopped wondering when he saw one move. He tried to point it out to Chmeee. The kzin ignored him.

The road curved away to antispinward as it approached a range of craggy mountains; it jogged through a notch in the range, rather than carving its own path, then jogged right to run alongside the swamp again.

But Chmeee swerved left and accelerated. The lander streaked along the portward side of the range, trailing a plume of fire. Abruptly the kzin spun the lander around, braked, and set down at the foot of a granite cliff.

He said, "Let us step outside."

The scrith sh.e.l.l of the mountain would block the Hindmost's microphones, but they'd feel still safer outside the lander. Louis followed the kzin.

The day was bright and sunny-too bright, as this arc of the Ringworld approached its nearest point to the sun. A stiff warm wind was blowing. The kzin asked, "Louis, were you about to tell the Hindmost of the Ringworld engineers?"

"Probably. Why not?"

"I a.s.sume we've come to the same conclusion."

"Doubtful. What would a kzin know of Pak protectors?"

"I know everything in the records of the Smithsonian Inst.i.tute, what little there is. I have studied the testimony of the asteroid belt miner, Jack Brennan, and holos of the mummified remains of the alien Phssthpok and of the cargo pod from his ship."

"Chmeee, how did you get hold of that stuff?"

"Does it matter? I was a diplomat. The existence of the Pak has been a Patriarch's Secret for generations, but any kzin who must deal with humans is required to study the records. We learn to know our enemy. I may know more of your ancestry than you do. And I surmise that the Ringworld was built by Pak."

Six hundred years before Louis Wu's birth, a Pak protector arrived in Sol system on a mission of mercy. It was through this Phssthpok, via the Belter Jack Brennan, that historians learned the rest of the story.

The Pak were native to a world in the galactic core. They lived their lives in three stages: child, breeder, protector. The adults or breeders were just intelligent enough to swing a club or throw a stone.

In middle age, if they lived long enough, Pak breeders developed a compulsion to gorge on the plant called tree-of-life. A symbiotic virus in the plant triggered the change. The breeder lost its gonads and teeth. Its skull and brain expanded. Lips and gums fused into a hard, blunt beak. Its skin wrinkled and thickened and hardened. Its joints became enlarged, offering a larger moment arm to the muscles, increasing their strength. A two-chambered heart developed in the groin.

Phssthpok came tracking a Pak colony ship that had reached Earth more than two million years earlier.

The Pak were in a constant state of war. Previous colonies to nearby worlds in the galactic core had always been overrun by subsequent waves of ships. Perhaps that was the reason this ship had come so far.

The colony was large and well-equipped, and guided by beings tougher and smarter than humans. It had failed nonetheless. Tree-of-life grew in Earth's soil, but the virus didn't. The protectors had died out, leaving a lost population of Pak breeders to fend for themselves ... and leaving records of a cry for help that had crossed thirty thousand light-years to the Pak home world.

Phssthpok found those records in an ancient Pak library. And Phssthpok crossed thirty thousand light-years, all alone in a slower-than-light craft, seeking Sol system. The resources that built that craft, in knowledge and minds and materials, were resources Phssthpok had conquered and held by war. His cargo pod was jammed with tree-of-life roots and seeds, and bags of thallium oxide. His own research had discovered the need for that unusual soil additive.

It might have occurred to him that the breeders would mutate.

Among the Pak a mutant stood no chance. If the children smelled wrong to their protector forefathers, they were killed. On Earth-perhaps Phssthpok counted on a lower mutation rate, this far from the savage cosmic-ray density among the core suns. Perhaps he took his chances.

The breeders had mutated. By Phssthpok's time they showed little resemblance to the Pak breeder-barring certain changes at middle age, when the production of eggs stopped in females, and when both s.e.xes showed wrinkling of skin, lost teeth, swelling of joints, and a restlessness and dissatisfaction that was all that remained of the hunger for tree-of-life. Later in life, heart attacks would result from the lack of the second heart.

Phssthpok learned none of this. The rescuer died almost painlessly, with no more than a suspicion that those he intended to rescue had become monsters, and had no need of him at all.

Such was the tale that Jack Brennan told to United Nations representatives before his disappearance. But Phssthpok was dead by then, and Jack Brennan's testimony was doubtful. He had eaten tree-of-life. He had become a monster; his braincase in particular was expanded and distorted. Perhaps he had become mad too.

It was as if a load of spinach noodles had been spilled all over this rocky area. Strips of greenery, fuzzy to the touch, hugged the ground in places where dirt had packed itself between the boulders. Clouds of insects buzzed around their ankles, staying within inches of the ground.

"Pak protectors," said Louis. "That's what I thought, but I've been having trouble making myself believe it."

Chmeee said, "The vacuum suits and the Gra.s.s Giant's armor show their shape: humanoid, but with enlarged joints and a face pushed forward. There is more proof. We've met so many hominids, all different. They had to be derived from a common ancestor: your own ancestor, the Pak breeder."

"Sure. It'd also tell us how Prill died."

"Does it?"

"Boosterspice was tailored for the metabolism of h.o.m.o sapiens. Halrloprillalar couldn't use it. She had her own longevity drug, and it could be used by a number of species. It struck me that Prill's people might have made it from tree-of-life."

"Why?"

"Well, the protectors lived thousands of years. Some factor of tree-of-life, or a subcritical dose of it, might trigger just enough of the change to do that for a hominid. And the Hindmost says Prill's supply was stolen."

Chmeee was nodding. "I remember. One of your asteroid mining craft boarded the abandoned Pak s.p.a.cecraft. The oldest man in the crew smelled tree-of-life and went mad. He ate beyond the capacity of his belly, and died. His crewmates could not restrain him."

"Yeah. Now, is it too much to expect that the same thing happened to some UN lab a.s.sistant? Prill walks into the UN building carrying a flask of Ringworld longevity drug. The UN wants a sample. A kid barely too young for his first dose of boosterspice-forty, forty-five-opens the flask. He's got the eyedropper all ready. Then he gets a whiff. He drinks it all."

Chmeee's tail lashed air. "I would not go so far as to say that I liked Halrloprillalar. Still, she was an ally."

"I liked her."

The hot wind blew around them, filled with dust. Louis felt harried. They wouldn't get another chance to talk in privacy. The probe that relayed signals to and from Needle would soon be too high up the Arch for this kind of trick to work.

"Can you think like a Pak for me, Chmeee?"

"I can try."

"They put maps all over the Great Oceans. Instead of mapping Kzin and Down and Mars and Jinx, can you tell me why Pak protectors wouldn't just exterminate the kzinti and Grogs and martians and banders.n.a.t.c.hi?"

"Uurrr. Why not? The Pak would not flinch at exterminating alien species, according to Brennan."

Chmeee paced as he mulled the problem. He said, "Perhaps they expected to be followed. What if they lost a war; what if they expected the winners to come hunting them? To the Pak, a dozen burnt-out worlds within a dozen light-years of one another might indicate the presence of Pak."

"Mmm ... maybe. Now tell me why they'd build a Ringworld in the first place. How the futz did they expect to defend it?"

"I would not attempt to defend a structure so vulnerable. Perhaps we will learn. I have also wondered why Pak would come to this region of s.p.a.ce in the first place. Coincidence?"

"No! Too far."

"Well?"

"Oh ... we can guess. Suppose a lot of Pak wanted to run as fast and as far as they could. Again, say they lost a war. Got kicked off the Pak world. Well, there was one safe route out into the galactic arms, and it was mapped. The first expedition, the one that settled Earth, got to Sol system without running into any danger they couldn't handle. They sent back directions. So the losers followed them. Then they set up shop a good safe distance from Sol system."

Chmeee mulled that. Presently he said, "However they came here, the Pak were intelligent and warlike xenophobes. That has implications. The weapon that vaporized half of Liar, the weapon you and Teela persisted in calling a meteor defense, was almost certainly programmed to fire on invading ships. It will fire on Hot Needle of Inquiry or the lander, given the chance. My second point is that the Hindmost must not learn who built the Ringworld."

Louis shook his head. "They must be long gone. According to Brennan, a protector's only motivation is to protect his descendants. They wouldn't have let mutations develop. They'd never have let the Ringworld start sliding into the sun."

"Louis-"

"In fact they must have been gone hundreds of thousands of years. Look at the variety of hominids we've found."

"I would say millions of years. They must have departed soon after the first ship called for help, and died soon after completing the structure. How else would all of these varieties have had time to develop? But-"

"Chmeee, look: suppose they finished the Ringworld a mere half million years ago. Give the breeders a quarter of a million years to spread out, with the protectors fighting no wars because the territory's virtually unlimited. Then let the protectors die off."

"From what?"

"Insufficient data."

"Accepted. Well?"

"Let the protectors die off a quarter of a million years ago. Give the breeders a tenth the time it took humans to evolve on Earth. A tenth of the time, and a lot of nice gaps in the ecology because the protectors didn't bring anything to prey on the breeders, and a base population in the trillions.

"See? On Earth there were maybe half a million breeders when the protectors died out. On the Ringworld, three million times the room, and plenty of time to spread out before the protectors died. The mutants would have it all their own way."

"I don't accept that you're right," Chmeee said quietly. "I do feel that you've missed a point. Granted that the protectors are almost certainly gone. Almost certainly. What if the Hindmost learns that this was their property, their home?"

"Oops. He'd run. With or without us."

"Officially we have not penetrated the secret of the Ringworld's construction. Agreed?"

"Yeah."

"Are we still looking for the Repair Center? The smell of tree-of-life might be deadly to you. You are too old to become a protector."

"I wouldn't want to. Is there a spectroscope in the lander?"

"Yes."