The Grooved Spheres

Over the last few decades, miners in South Africa have been digging up mysterious metal spheres. Origin unknown, these spheres measure approximately an inch or so in diameter, and some are etched with three parallel grooves running around the equator. Two types of spheres have been found: one is composed of a solid bluish metal with flecks of white; the other is hollowed out and filled with a spongy white substance. The kicker is that the rock in which they where found is Precambrian - and dated to 2.8 billion years old! Who made them and for what purpose is unknown.