5. The Antikythera mechanism.
This was fished out of the sea many years ago and appears to be a device so complex that it's often said that nobody could have made it at the time.
Evidently, someone did.
There are people in the world today who could start from a sheet of brass and make an accurate watch mechanism. Not very many, but there are some. Nowadays few would bother because many of the cogs and wheels are already available as mass-produced parts, but armed with a few simple tools and a lot of time and skill, it could still be done.
It accurately charts the planets, the moon (including correcting for the moon's elliptical orbit) and performs a whole range of astronomical calculations. All of them accurately. So, is it some mysterious time-travel artefact or a design passed on by aliens?
At the time it was built, astrology was a very serious matter indeed. A device that could accurately predict what the sun, moon and planets would do in the future was a highly desirable thing. It would be something that someone, possibly royalty, would pay a lot of money for. Therefore it was certainly worth some skilled artisan's time to cut all those cogs and gears and make sure the whole device worked.
How did he predict the moon and planetary movements? He didn't. He observed them and replicated the movements in his device. Since each object follows a predictable path, the historical data available to the device's maker would extrapolate very nicely into future data. All he had to do was make sure it gave accurate results for any point in the past, and it would give accurate results for any point in the future.
Easy? Certainly not. Planets don't follow neat circles around the sky, certainly not when seen from Earth. Sometimes they even appear to go backwards for a time. Getting all that into the machine would need a lot of cogs and wheels and that's exactly what this device has.
It was made by someone exceptionally clever, highly skilled and who had time and resources to do it. Most likely it was commissioned by someone very rich, someone who could keep the maker employed solely on this job.
Are there others? Well, you wouldn't pay someone on the promise that they could make something unless you'd seen previous work. So simpler mechanisms were probably produced along the way. As this site says, the Antikythera mechanism contains no errors so it's likely there were prototypes.
Why haven't we found any? Consider how long it takes to cut one accurate cog starting from a sheet of metal. When you have a prototype but need to change some things, you keep all the parts you've made that already work. I'd say the prototypes were most likely cannibalised for parts so the chances of finding one of them will be very slim now.
As for it being the only one, well, it would have taken many years to get this one right and there was no such thing as mass production. It might well have been the only one ever made, especially if the maker or his rich sponsor hid or destroyed the plans and prototypes. Having the only accurate planet-predictor would have been a very powerful thing in those days.
I don't see anything supernatural or inexplicable in this device. It's fascinating, certainly, but it's not beyond the bounds of human ability for the time. All it would need was the backing of a rich sponsor. It would also take someone most of a lifetime to get it right, so finding only one isn't a surprise either.
So it's not inexplicable at all.
The next one might be.