Stonehenge and the those huge heads on Easter Island are the best-known of the world’s greatest megalithic mysteries. They’re certainly not the only ones, however. In Costa Rica, for instance, curious stone spheres like the ones pictured above have kept scientists stumped for almost a century. Las esferas (spheres), or bolas de piedra, as they’re known locally, range in size from a few inches in diameter to nearly nine feet around. A good 300 of them have been found to-date, with the largest ones weighing upwards of 26 tons! Like the Easter Island Moai and Stonehenge structures, no one is enti...