A visitor from beyond our galaxy (maybe from beyond anywhere) Buddha's Wheel passed Earth's solar system in the year 2408 at a distance of half a lightyear.
Traveling at a relative snail's pace of three thousand kilometers per second, it was first detected by the ultra-long-wave radar network in the Kuiper Belt.
It took another two decades for Earth's fastest antimatter vessel, the Arctica, to catch up with it in the cold of deep space.
Before the encounter, the improvised human/hybrid crew (38 homorphs and 96 anibots) prepared for every conceivable contingency.
It's impossible to prepare for the absolute unknown. One has to make an effort though.
We found a centripetal-wheel spaceship 110.7 kilometers wide, spinning to generate 1.7 g of interior gravity. There were no hubs or spokes, just a slowly spinning metallic circle.
Only fifteen degrees above absolute zero, the exterior of the great wheel (or ring or hoop - though the middle wasn't as hollow as it first seemed) was completely featureless, except for the row of identical scoops lining the outer rim.
Each scoop was large enough to hold all the ships that have ever sailed Earth's seas.
Rolling toward its unknown destination, the Wheel had to be vastly more advanced than it appeared. The first clue was its unblemished outer surface.
In fact, it was so advanced the small crew of humanoid visitors which spent less than two months inside was utterly irrelevant to its existence. We were beneath beneath contempt.
Everything we think we know about its builders must be wrong:
Created by an "Undergod" Class-4 super-civilization, the Wheel may be part of a billion-year plan. If their history ended eons ago, their only remaining goal must be perfection. Sufficiently advanced civilizations may decide to cut themselves off from the rest of the universe forever.
After discovering the deep secrets of physics, the Undergods' future had become as clear to them as the past. Their Wheel might use quantum feedback amplification to plot its path through reality. Its very passage preempted any threats before they could occur.
That was why we could enter and explore 6% of its interior without consequence. As a side effect, it may even have stabilized human civilization before its passage.
Arctica managed to touch down on the inner rim in an elegant deceleration maneuver.
It took another three weeks to figure out how to enter through one of the mysterious scoops. This led to a claustrophobic, gel-filled airlock, which led to the Sponge Maze.
The outer rim of Buddha's Wheel was over one kilometer thick, and almost as strong as solid diamond.
It was riddled with several millions of kilometers of narrow access shafts and corridors leading every which way. Between them were smaller tubes and tunnels of all imaginable shapes and sizes, down to nanometer scales and below. Some glowed like fiberoptic stars, providing the dim illumination.
The atmosphere was a complex mixture - 8% oxygen at 2.6 atmospheres, most other gasses being synthetic molecules. We never found two molecules that were exactly alike.
Not even Earth's toughest bacteria could have survived in that mixture for long, but our exosuits never sprung a leak.
The first wave of exploration bots spread out for hours, climbing and descending the strangely curved tubes connecting the different levels, before finding their way back to the Entry Room.
Since radio didn't work beyond line-of-sight, the bots had to defragment into hundreds of pieces each, running back and forth to keep us updated.
There were many doors and gates that could slam shut at any time. It felt like an epic journey, but they never got further away than 175 meters.
Because of the smooth walls and absence of corners, we suspected the tunnel network was meant to transport fluids. We didn't stay too long.
The next day, the bots found a path to a spiral tunnel rising to the interior surface, more than a kilometer above our heads.
It led to the Oculus, an airlock made of sliding panels.
When we walked out onto the main floor and looked up, it was like a caveman visiting a human space station. Human aesthetics, the rules of symmetry, complexity, and redundancy didn't apply. Nothing was truly random. Everything had evolved to its ideal state.
We stood inside a curving tunnel ten kilometers wide and high. Almost flat on the bottom, at least for the first few hundred meters. Then the ground seemed to rise up in both directions, steeper and steeper, to finally vanish behind the sky. Both ends of the tube, spinward and antispinward, looped back together 108 kilometers overhead.
Harsh, violet light glared from the artificial sky.
A standard Von Braun space station - only one billion times bigger. Big enough to have weather. The clouds were disturbingly regular, like kites or dirigibles.
The ground was covered with something like blue felt. Directly ahead lay our first challenge, one of the most intimidating sights ever seen by human eyes.
The Black Jungle was a teeming, steaming wilderness; chaotic terrain with steep, jagged hills and deep, misty valleys.
Up close, the densely packed plant life turned out to be made of delicate machinery of every imaginable variety, without discernible purpose. Even the sponge-like moss was made of millions of perfectly interlocking levers and pistons in a web of wires, with not a nanoscopic cog out of place.
Fortunately, the large entities we sometimes heard moving in the distance never came closer. It took rather long to figure out why.
It turned out the chaotic wilderness was actually a single integrated device, perfectly organized to perform one unimaginable task. Instead of a wild junkyard, it was the summit of intelligent design.
Things only got stranger.
Beyond the Jungle, the curving tubeway was filled with great hanging ribbons of all sizes. Behind them, we could just see what looked like a series of oddly asymmetrical temples, leading to a tunnel of light.
Long-range scans revealed several physical anomalies within this well-lit but apparently empty area. The absence of background neutrinos from that direction indicated it wasn't nearly as empty as it looked.
Buddha's Wheel had a complex artificial climate, with many types of synthetic rain and snow.
Once, we were caught inside a bright sparkling storm, like being submerged in analog static. Afterwards, the environment had changed, with new colors and layers all around us.