Nothing important had happened for a billion years, and Rulon Kinn was getting restless again. The political maneuvering and intrigues of the Polis continued unabated. The age-old Celvus dynasty was finally overthrown by Dibo I, who sent his mounted troops to capture every boulevard of the Polis. Rulon joined the struggles on the barricades, knowing the troops couldn't kill or maim anyone, only hurt them - which might be worse. It wasn't easy being immortal.

  He searched the Polis for others like himself, but few were similarly afflicted. Lan Uret appeared to be another self-selected outcast. Both belonged to groups that affected pre-Polis fashions, spoke reconstructed ancestor languages, and spent their time reading ancient texts. Lan was more practical and decisive.
Under the new regime it was best to keep their opinions to themselves. They moved to a remote suburb, near the endless wastelands surrounding civilization.
After a few decades their bold plan took shape.

  "Let's go for a walk," Rulon said.

  "Right now?"

  ~~~

  The universe was an endless maze.
Leaving the Polis through the Great Plain, the Valley of the Ancestors, and onward into the Northern Funnel, they finally entered the Tunnel Network.
A person could walk literally forever down these smooth, empty corridors. The walls appeared metallic, without perceivable texture or temperature. Their reflections flowed as they moved, but nothing else changed. The air itself glowed slightly as in the Polis.
A constant breeze equalized the pressure between adjoining domains, a slight whistle that occasionally became a wild storm.

  Standard physics didn't apply anymore. They could circle around a curving hallway and emerge inverted as their own mirror images. Even their senses were inverted, making it appear as if reality had flipped.
Portals and gateways led to endlessly diverse landscapes. The few previous explorers had reported strange plains extending in all directions, sometimes impossible directions where it could take twice as long to turn around, and impenetrable forests where the trees seemed to change shape from different angles.
The sky was usually a mirror reflecting the terrain below.

  The internal dimensions of each pocket universe were perpendicular to all the others, Rulon reasoned, meaning each was infinitely large. If they wished they could continue exploring forever.

  Lan questioned his odd theories, but he had all the time in the universe to refine them. Slowly Rulon persuaded her.

  "Everywhere our people have visited, conditions are hospitable for life," he said. "The universe must have been specifically designed to allow us to exist. I need to know why."

  "Sounds like reverse causality."

  "If so, there must be an origin point - the Wall of Non-Existence or Time's Edge."

  Rulon studied the ancient legends for clues. They claimed the universe and its inhabitants had been created within a lost supernatural realm. This "Earth" universe was round, not flat, yet people somehow clung to its surface (if only briefly). Strangest of all, Earth wasn't connected to anything, but was surrounded by sheer nothingness, if such a concept could have any meaning. How could Nothing exist without air to fill it? His mind would not accept it.

  "I don't see how you could prove the existence of something outside our universe," Lan said.

  "With a transfinite proof, of course," Rulon answered.

  Thanks to the Polis's marginal birthrate, only a few nearby plains had been settled. People could get lost one hour's walk from the final outpost.
That would never happen to them, Rulon assured Lan. With his flawless recordkeeping and her uncanny perception, they could always retrace their steps no matter how far they ventured.
She submitted to his scheme.

  Climbing down a narrow hole in the floor, they entered the first unexplored tubeway. There was a moment of vertigo as they found themselves inverted, and then they were standing on what should have been the ceiling. The light seemed deeper here, like the colors of ancient paintings.

  Rulon made only one assumption: that the geometry of the universe remained consistent. If he was right, they would always be able to backtrack, though he hoped that wouldn't be necessary.
He intended to walk a perfectly straight path until they made their way back to the Polis: a Grand Tour of the universe.

  Strange echoes preceded them through the metallic hallways. Sometimes the corridors widened into caverns within the space of a few steps before contracting again. Once, they found the decayed skeleton of some unknown creature, each bone the size of a house. Not all life was immortal. It couldn't have been here for long. Unmoving matter was reabsorbed by the walls in a few millennia.

  The first new landscape was like the Polis Territory, but it had never been settled. Endless lush valleys stretched ahead. Just below the sky ceiling, swarms of colored balloons chirped like immense birds. In one valley they found what looked like a mountainous egg, throbbing softly. They gave it a wide berth.
After only thirty years, they found a concealed passage into a different set of steel tunnels.

  Lan effortlessly remembered each turn they took. In this respect her memory was flawless, part of the structure of reality itself.

  One gateway led directly into an ocean, the surface a shimmering membrane against the side of the tunnel. Another portal opened in featureless sky, with only the sound of distant thunder when they poked their heads through.
Finally, they reached a gateway so large it filled the corridor.

  "I suppose we have no choice," Lan said.

  This time the vertigo was overwhelming, the closest to pain either traveler had ever known.
This realm had no sense of scale. They stood under immense blades of grass casting shadows hundreds of meters long. The sky ceiling was too high to see.
Lan noticed what looked like a carpet of tiny trees underfoot.
Climbing halfway up a wide blade which didn't even buckle under their weight, they studied the overpowering panorama. The clouds were white pillars twenty kilometers high, row after row vanishing to infinity. There was a ghostly hint of even greater sky mountains above and beyond them.
These rather resembled the puffs of fog drifting between the great blades, seeming to shrink and multiply closer to the ground.

  "This might be Waltisir from the Zyther legends," Rulon said. "The next gate will be billions of kilometers away."

  They crossed the endless jungle, years merging into decades. Time meant little for their consciousness. Days passed effortlessly and vanished forever. They hardly deviated from their course, relying on Lan's navigation to walk a straight line.

  "What in Earth's name is that?" Rulon asked one day.

  Indistinct through the distant haze, a many-legged shadow clambered up a mountain flank, then leaped to the next mountain as if it were a piece of crumpled paper. The creature looked like one of the small termite-weavers they saw from time to time.

  "Scale invariance," Lan said.

  "I just hope we don't end up stepping on ourselves."

  Near the horizon, the ground shimmered like a mirage, surface shockwaves approaching at tremendous speed.

  "Let go of everything," she said.

  The ground wave crossed the forests and fields too fast to see, and they were flung into the sky. The vibrations broke several older strands of grass, which collapsed to the ground with tremendous force.
It took another hour for the sound of the monster to reach them. Avalanches rolled down distant mountainsides, a black wall of dust that settled before it could reach them. A new hill had been raised in the distance.

  "If that thing comes our way," Lan said. "I don't care if we're immortal."

  A few millennia later they saw their first dragon, soaring impossibly high, casting subtle shadows for hours.
Near the end, they crossed an interminable plain of very short, almost felt-like grass. There were no landmarks on the horizon. Half the universe was green, the other half light blue. Finally, a treeline appeared in the distance.

  Not many millennia later, in a particularly dense thicket of undifferentiated branches, they discovered a smooth crater. After excavating the accumulated layers of ancient mulch, hummus, and assorted debris from the bottom, they found themselves staring into a new sky.

  "We followed the Long Line," Rulon said. "This portal will take us home."

  They pushed through into a land of small trees and low hills. The terrain looked strangely familiar, but it felt wrong. The air seemed unpleasantly bright, or perhaps too evenly lit, free of shadows.
The effect passed, and it began to get darker.

  Lan asked, "What legends mention this place?"

  "None directly," Rulon admitted. "This may be 'Endland'. No one returns from here."

  "At least we made it this far."

  They walked for a long time. As the air seemed to thicken, darkness creeped into every surface and under each blade of grass. Rulon knew Lan suspected something, but he wouldn't ask. His pride had multiplied during their trek.

  Rulon found he could actually see the blackness, like a virtual haze, even when he closed his eyes. Visibility dropped to a few meters.

  "I think we finally found something that could kill us," Rulon sighed. "We'll have to take the long way back."

  Lan said, "But we're almost home."

  "Impossible! We've never been further away!"

  "It's not the land that is strange, it's us."

  It was hard to read Lan's expression in the darkness. He should have guessed sooner.
"We've been inverted again, but not in space. Are we moving backwards through time? No, not quite . . . at a 161 degree angle and rising fast."

  She kicked a rare Purpleberry lying on the ground. It shot up, and gently attached itself to an overhanging branch. "You tell me. If we wait long enough, we might return before we left! Probably not - but I want to see what's beyond the next gate."

  Rulon thought harder than ever before. "This solves the age-old thermal problem. Everywhere we go, there's ample light, and the atmosphere has a constant temperature. Where does the energy come from? Why doesn't the heat accumulate, frying us all?"

  "I've read about the entropy problem," Lan added. "Why doesn't the universe run down and stop?"

  Rulon nodded. "Constant energy is endlessly recycled through the tunnels. Our bodies operate on a similar principle, which is why we live so long. Apparently, the people on Earth had to eat almost daily."

  "They must have perished in huge numbers."

  "To eliminate the accumulated disorder, matter sometimes appears to go back in time. It's built into the geometry of the universe, forever recycling in a Steady State."
The darkness increased marginally, chilling the air a fraction of a degree. "Since it appears perfectly stable, our universe may have always existed. When was the Polis founded?"

  "No one is sure," Lan said. "Our brains are too small to count that high."

  "So it's ageless. It just keeps cycling through the same finite number of events. Eternal recurrence: we are born again, relive our lives and all other lives, and die after many eons. Our universe was not created. It just is. Perfection sustains itself."

  Lan asked "Does that mean Earth doesn't exist?"

  "That legend must have been a fabrication like the Sythili Chronicles. Maybe every possible world exists somewhere, but if so, Earth has nothing to do with us. Now comes the final question: will we survive the next portal?"

  Lan stared timelessly. "We won't remember our journey if we make it back."

  "That settles it. We are not returning to the Polis. The only way to stay ourselves is to keep heading outbound."

  "If we don't return, how can there be endless recurrence? Our atoms won't be recycled."

  Slowly Rulon began to turn, willing the river of time to change course. "Eventually, others will be born just like us. They will relive our lives to the last detail."

  "Then they'll go on this expedition too."

  Rulon pointed at a nearby rock. "They already have."

  There, barely visible in the stone, were their worn initials, ancient beyond measure.

  She grabbed a sharp rock conveniently lying nearby, and said: "Time to touch it up a bit."
He felt the beginning of strange new emotions as he looked at her.