Stone balls. It is easier not to notice them, or to come up with some kind of story in the likeness of ancient stone money or something like that.
Stone balls in Costa Rica were found in the thirties of the XX century by workers who cleared the jungle for banana plantations. At the same time, many petrospheres were split, as workers, remembering local folklore, began to look for gold in them.
Archaeologists became interested in the mysterious balls, they began to be transported to museums; in our time there are almost no petrospheres in the places of their discovery. In total, about three hundred balls of different sizes were found, the largest reaching two meters in diameter and weighing 16 tons.
Dating the petrospheres is also difficult. Presumably, they were made in the period from 1500 BC. to 200 BC However, the balls may be much older. The purpose of the petrospheres is unknown.
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Scientists admit that the balls could serve as a designation of territorial boundaries or were symbols of heavenly bodies. For some time it was believed that the stones acquired a spherical shape under the influence of water, that is, in a natural way. This explanation has now been refuted.
And of course, there is a fantastic hypothesis that ascribes the creation of the balls to aliens from outer space who once visited Earth.