In Costa Rica there are over 300 mysterious spherical stones like this of which nobody knows the exact origins. I, of course, think they came from aliens, however, some of the more common myths around their creation include that they came from Atlantis, or that they naturally occurred. There are also local legends saying that some native inhabitants had a potion to soften rock. They range in size from a few centimeters to about 6 feet in diameter, and some weigh tons! The date range for most of them doesn't really narrow their age very well - 200 BCE - 1500 CE.
The first time they were recorded is around 1930 when the United Fruit Company came across about a dozen when they were clearing the jungle for bananas (booo- they have been accused of bribing government officials, paying little taxes to the countries they exported from, exploiting workers, and trying to create a monopoly in most of the countries where they harvested). When they were originally found, they were often grouped in alignments that have been speculated to relate to the magnetic poles or astrological phenomena.