Perfectly round stone spheres. Thousands of them. Where do they come from?

They are perfectly simmetrical.
They sometimes appear partially buried, as if they have been 'launched" from above.
They have appeared in Costa Rica and Bosnia and many other sites.
A simple search on Google images will give you many photos to look at.

Here are a few, if you find more please share the link with us.
Source: http://www.realufos.net/