From high above and afar, after the planetary shell opened a cleft to let the Tahirian spacecraft through and closed again behind, the spectacle was awesome enough. White steam and black smoke roiled in upward-rushing winds where sometimes flame flared. Below sprawled and reared a step pyramid the size of a small mountain, bearing towers, battlements, portals, keeps, roadways, trackways, kilometers-long tubes of mighty bore, forms as alien to human eyes as functions were unknown to human minds. Around it spread a forest of lesser structures, dense near the center, thinning out with distance until empty desert framed the edge of vision. They bore many different shapes, but dominant was something like a metal tree with an intricate mesh between the leafless boughs. Lights flashed throughout, a tumbling, bewildering shift from moment to moment, so that the men caught illusory half-glimpses of fireworks, waves, a maelstrom, a thing that danced. Machines scuttled about or clustered to do some task. Here and there moltenness welled up, seethed sullen red, rolled slowly down channels until it congealed into dark masses. There the machines were at their busiest.

"Jesus Christ!" Brent rasped. "What is that? Like the middle of hell!"

"I — I think I can guess," Cleland said.

"Better wait till we have had a closer look," Nansen advised. The spacecraft sped onward and the titans' workshop sank below the horizon.

A number of the third planet's fifteen-hour days passed before the three visitors saw the sight again. Then they were not sure whether it was the same one; there were several, distributed over the globe. Although they and the Tahirians could now communicate slightly, most things remained obscure, occasionally even the interpretation of a map. This particular uncertainty didn't matter. They had already encountered more astonishments than they could sort out.

The tour began at an enclosed headquarters where air was breathable, with imagery of the original work. Comets had not contented the builders; to win the stuff that was to become atmosphere and hydrosphere, their machines also dismembered an icy moon of a giant planet and put the pieces on a collision orbit. Centuries later, when things had somewhat quieted down but not much gas had yet escaped back to space, they roofed the world. Thereafter they tapped its own buried reserves of ice, though that was a minor contribution.

"Tremendous!" Brent said. "We've got to learn the engineering. What we could do with it —"

"I wonder," mused Nansen. "Will humans ever start anything that will take millions of years to finish? It's a rare man who tries to provide for his grandchildren."

"He can't," Cleland replied. "Human affairs are too, uh, chaotic. Everything's bound to change b-beyond recognition in less than a thousand years. Nothing's predictable. The Tahirians, they've achieved a ... stable society. And any, uh, profit motive is irrelevant, when self-maintaining, self-reproducing robots do the work."

"Um, why was the work ever begun? What need for it?"

"Never mind now," Brent said. "I'm thinking what we could do, we humans, for our purposes, with power like this."

"We may well find it waiting for us when we return," countered Nansen.

"Or we may not. Or if we do — we won't arrive helpless."

A pillar, one of those that upheld the enclosing sky, was in many ways still more numinous. No plain shaft, it rose organismically intricate, responsive, well-nigh alive. Its interplay with strata and height, ground tremors and winds, not only kept it standing but acted on them, was a force in the development of the planet. The ascent and descent of it, in a bubble that climbed, with stops to go inside for an uncomprehending look, became a voyage in itself.

The waters teemed with microscopic life. Some had begun to aggregate: patches of scum, ragged mats. Crumbling shorelines revealed where germs gnawed rock down to the motes that would go into soil. The life had not arisen naturally, nor was its evolution driven by blind chance and selection. An underlying, interweaving set of mechanisms — biological and nanorobotic together, Cleland speculated — would hold it on the many roads to its destiny, which was to prepare this world for the kind of life that lived on Tahir.

"Everything guided," he muttered. "Accelerated. Maybe half a billion years will serve. Maybe less. Though if they want what they've made to last as long as that —" He stared before him. "Yes, I believe I'm right about what those monster establishments are for."

And at last their guides, the Tahirians they called Emil and Fernando, brought them to one.

They stood a long while by the aircar, trying to grasp a sight so huge and strange that their eyes did not know how to see it. At their backs reached the barrens, red dunes, black rock, scudding dust. On the right a pillar lifted white over the horizon, made thin by distance, sheering and narrowing until it became a point thrust against the violet heaven.

Ahead lay the forest, metal boles, skeletal limbs, glimmery webs from end to end of vision. Shapes hunched among them, low domes, pentagonal upthrusts, helices, congeries. Robots moved between, some like giant beetles, some like dwarf war machines, some like nothing describable. The forest thickened farther in until it became a shadowy mass. Beyond loomed the central pyramid, its terraces saw-toothed with walls and spires. Smoke and vapors hazed it to blue-gray. Everywhere blinked the light-sparks, over the metal trees, tangled in their meshes, all hues, chaotic in complexity, the lunacy of a million intermingled meanings. A rumbling noise filled the air, a bass that stole into the bones. Now and then the ground boomed and faintly shuddered.

Nansen brought his glance back to the Tahirians. Emil made a forward gesture. Evidently it was safe to proceed on foot. They started. Human pulses beat high.

In a gravity less than two-fifths terrestrial, the men did not feel weighted down, although they were well burdened. Elsewhere it had sufficed outdoors to wear a breathmask attached to a tank of oxynitrogen, ordinary field garb, perhaps a canteen and food pouch, and whatever scientific equipment seemed appropriate. Here it became helmets with regenerative units, full coveralls, gloves, and boots. The air was noxious from upwellings and ongoing chemistry. The Tahirians wore similar protection, though theirs was mostly transparent and flexible, films enclosing the little four-legged bodies. Fernando carried what must be a locator. A magnetic sense was of scant avail on this planet, and the forest would screen off signals from their vehicle.

Minutes and meters passed. A machine composed of joined modules crawled into view. Emil spoke. Sonic amplifiers conveyed a whistle and purr; the mane shook.

"I wish I knew what en just said," Cleland sighed. Sundaram had invented the pronoun for members of the race. "That caterpillar  —  It's maddening, these scraps of pidgin technicalese we've got."

"We've added some vocabulary on this trip," Nansen reminded. "And Ajit does promise a real language soon." He grinned. "Of course, then we'll have to learn it."

"Meanwhile, though — And we can't ask what all these lights are for."

"I can guess," Brent said. "Whatever else they do, I'm pretty sure the trees make up a sensor-computer network. The blinks are a code, mainly issuing orders to the mobile machines."

Nansen nodded. "M-m, yes, that sounds like a very Tahirian idea."

"I wish it weren't," Cleland complained. "Confusing. Makes everything seem to jump around. Do you hear a buzz?"

Nansen listened. Only the noise ahead, as of a gigantic kettle aboil, reached him. "No."

"Imagination. I'm getting sort of dizzy, too."

"Yeah, can't say I feel comfortable here," Brent admitted.

Nansen scowled at the stiff shapes that surrounded them. "Nor I, quite. Perhaps we should turn back. . . . No, we'll continue for a while."

The growling and seething waxed. The group came in view of the source. Cleland jarred to a halt, stood for a moment agape, and uttered a yell. "It is! I was right! It's got to be!"

They had entered a broad open space, a black and jumbled lava waste. Near the middle squatted a sooty shell wherein power labored. Before it, smoke and steam eddied above a ten-meter pool that glowed red. Even at their distance and through their suits and helmets the men felt its heat. It bubbled, spat gouts and sparks, roared in its fury. A channel bore the molten rock off, to congeal after a while into slaggy masses. Several robots quarried these as they cooled, loaded them into a great open-bed vehicle that rested wheelless above the ground. Metal trees lined the channel. Their lights blinked through the same intricate rhythms, over and over.

Steady state, Nansen thought. Repetition, except when something goes wrong and the robots make repairs. A full carrier takes the freight away and dumps it, while another arrives for more. Through millions of years?

The din hammered in his skull. He felt suddenly as if he were falling down a chasm. Tensing every muscle, he hauled his gaze free of the hypnotic flash-flash-flash.

Cleland's word's clattered. "Yes, yes, the pyramid, it holds the magma pump, electromagnetic or however it works, but here's one of the outlets, and maybe before the stuff's left to weather the pyramid processes it for calcium, phosphorus, potassium, or maybe erosion and biology do that well enough, but here's the thermostat, safety valve, renewal —"

He doesn't sound like himself, Nansen thought vaguely. What's the matter with him? He recalled the planetologist's controlled excitement of days — weeks? — ago:

"Plate tectonics keeps Earth alive. It frees the elements that life locks up in fossils, and releases others like potassium. It raises new rock to take up carbon dioxide as carbonate, and takes the old rock down below before the carbon dioxide is too depleted. This planet the Tahirians are converting, it's got to have that cycle or the atmosphere will never be right for them, but it doesn't have subduction. Also, I suppose, with a shell enclosing it, they don't want enormous shield volcanoes. But then they've got to bleed off the core energy; and they can use it for more of the geochemistry they need, eventually for stabilizing the air and regulating the greenhouse. I think they're making a start on doing all this artificially, and in the long run making it natural, self-operating for the next couple of billion years. They've drilled clear down to the core. The mills of God!"

Today:

"Science in action," Cleland chanted at the lava well and the machines. The ground drummed, the wind hissed, the lights flickered and flickered. "Oh, what's to learn!" He rocked forward across the stone field.

"Something's damn wrong," Brent groaned. "I've got a headache to kill me."

"And I — vertiginoso —" Nansen looked to Emil and Fernando. They stood calmly, innocently, untouched.

Cleland kept going. Did the Tahirians show concern? They glanced from him to the captain. Leaves stood stiffly in their manes. They twittered.

"Cleland!" Nansen shouted. "Come back! That's dangerous!"

The planetologist tripped on a lump. He fell, rose, lurched on toward the channel where the trees flashed.

"Hey, is he out of his head?" Brent cried. A fire fountain leaped briefly up from the pool.

"I... don't know — Cleland, Cleland!"

We're giddy, muddled, like drunks. What to do, what to do? "Go get him," Nansen begged the Tahirians. They stared back, obviously worried but baffled, unwilling to take action . . . because the man must know what he was doing. . . .

He doesn't.

The knowledge burst over Nansen. He swung around, seized Brent by the shoulders, wrestled the engineer about until they both stood sideways to the lava stream. His brain still gibbered, but it became an undertone. Listen," he said fast. "Those lights combine to flicker frequencies that cause something like an epileptic fit — in humans. I remember reading about it. He's lost all judgment. He's going for a close look, and probably he'll fall into the channel, the lava. I can't explain to our friends and ask them to go after him."

"Shit in a whorehouse!" Brent exploded. He pulled away from Nansen's grasp. "I'll fetch him."

"No. I will."

Brent glared. "Like hell. You're our captain."

"Yes, I am." And the commander sends no man into danger he would not enter himself. "But you work with machinery, You can better gauge distances and angles. I'll go blind, eyes shut. Else the lights might take me, too. You call directions. Don't look at them more than you absolutely must. Can you do that, Mr. Brent?"

The other man snapped to attention. "Yes, sir."

"Very well." Nansen wheeled. Before the lights could touch him again, he had squeezed his lids together.

Cleland was already near the channel bank. He slipped and stumbled in the chaos of scoria. It got worse the farther he went.

Nansen strode. "Left about ten degrees," came Brent's voice, faint beneath the thunders. Nansen tuned up his amplifier. The well brawled. The formless blobs of blindness floated before him.

But his mind was clearing. Out of the racket he sifted words: "Right just a tad. . . . Watch out, you've got a boulder in your path. . . . No, Tim's veered off. Bear left about fifteen degrees. . . ."

He's the hero — Brent. He clings to his sanity and forces his judgment to function, while the lights flash, flash, flash.

Could there be something else, some inductive effect on our brains from the computers everywhere around, making us this vulnerable, Tim the most? I don't know. I hope we'll live to know.

Nansen tripped and fell. Pain jabbed through knees and hands. He picked himself up, he groped onward. Heat washed around him and smote through his helmet.

"Sir," amidst the tumult, "he's climbing onto the bank. You'd better turn back."

"No."

"But you might go over, yourself, into the stream —"

"Guide me, Mr. Brent."

"Sir. Left, ten degrees. No, a little more right. Careful, there's a heap of junk ahead. . . . Better go on all fours."

Nansen obeyed. The heat in the rock baked through clothing and gloves.

Dimly in the roar: "You're nearly to him. He's on the rim, staring down. You could knock him over, if he doesn't fall on his own account first."

Nansen opened his eyes. He saw the lava up which he crawled. And two boots, ankles, shins  —  He surged to his feet. "Come back with me, Cleland," he said into the noise and the scorching.

The enthralled man made no answer. He swayed where he stood above moltenness. Again the lights attacked Nansen.

He laid hold. Cleland moaned. He struggled. Nansen got a lock on his arms, a knee in the small of his back. He wrenched him around.

The lights were behind them. Nansen saw only stone, the forest that was merely befuddling, Brent and the Tahirians at its edge. Emil and Fernando stood stiff — suddenly realizing what had almost happened? Nansen frog-marched Cleland down the bank and across the waste.

"Hey!" Brent shouted. "We did it! By God, we did it!"

Cleland slumped. "Wha — wha's uh matter?" he wailed. "I was — I don't know —"

Nansen released him but kept an arm around his waist. "You'll be all right. Come along. And never look back to where you were."

"Cap'n — Cap'n, was I crazy or, or what — ?"

"It will be all right, I say. We simply met another thing we did not know."