3. The Giant Stone Balls of Costa Rica.
Huge carved stone balls, some six feet wide, litter the ground of parts of Costa Rica. In a world where humans have built Stonehenge, the pyramids and the carved heads of Easter Island, do these even need any explanation?
There is a quarry, some fifty miles from where some of the stones were found. How did they get there? Well, I would suggest that if you have several tons of rock, moving it from one place to another would be difficult if it was shaped like the rectangles of Stonehenge, the heads of Easter Island or the shaped blocks of the Egyptian or Inca pyramids. Stonehenge's bluestones, in particular, were moved hundreds of miles from their source to the place of construction.
By comparison, moving a stone shaped like a sphere would be a simple job indeed.
How were they cut so accurately? Actually, they aren't perfect spheres although they look pretty impressive to the eye. To check them for roundness during construction, all you need is a critical eye and a length of rope. Put the rope around the rock, note the length, put it around in a different direction and if it's a different length, it's not a sphere yet. More chipping away required.
The amount of chipping, and the fussiness of the builder, might explain why some of those balls are quite small. Some carvers just couldn't stop until it was exactly right.
In a society where nobody has access to laser-sighting equipment and giant calipers, a ball that looks round is good enough to impress the neighbours.
And that, most likely, is what they were for. Humanity hasn't changed all that much. Now it's all about who has the biggest, newest, fastest car or the latest fashionable shoes or the right logos on their T-shirts. People were no different in the old days. On Easter Island they competed for the most ornate stone head. In Egypt, the Pharaohs tried to out-do previous pharaohs with a bigger or more internally complex pyramid. In the UK, one village put up a carved rock, the next village put up a bigger one, the next put up a circle, the next placed huge rocks on top of other huge rocks... and in Costa Rica, they made bigger, rounder stone balls and moved them further than their neighbours.
I mean, really. Aliens? An alien race goes to all the trouble, time and expense to cross the vast gulf of space, makes some huge granite marbles and goes home again? Seriously?
Look at sculptors, ancient and modern. Many living sculptors could produce such a stone ball now, and with modern tools they could make a bigger and rounder one than any in Costa Rica.
These are interesting ancient artefacts. Whether they were part of some religious rite or just the ancient equivalent of garden gnomes, we'll never know. However, they were made by people.
No mystery here at all.