Klerksdorp spheres are small, spherical to disc-shaped objects that were found in 3-billion-year-old deposits in South Africa, and they've sparked a lot of mystery and debate.
Pseudoscientists claim they must have been made by intelligent beings because they seem so out of place.
Geologists say they're just the result of natural processes.
There are other mysterious historical finds too, like the Antikythera mechanism, which is an ancient Greek analog computer, and the Baghdad Battery, which might have been used to generate electrical current thousands of years ago.