Some of the human artifacts aliens will find are probably going to be as perplexing to the extraterrestrials as they are to us, unless they have some kind of time-vision technology. That would come in handy for unraveling the mystery of the stone spheres of Costa Rica. First discovered in the 1930s by the United Fruit Company, these perfectly-formed stone spheres range from the size of a basketball to one nearly seven feet in diameter that weighs fifteen tons. Nobody knows how they were shaped or what they were used for, and we may never know.