The Riddle of Ancient Roman Concrete, David Moore, PE
While shot blasting a coal seam in 1928, a miner found, among the dislodged coal, blocks of concrete about a foot across. The faces of the blocks were highly polished. The remainder of the wall disappeared into the coal seam. A second miner working a coal face about 100 yards away struck what seemed to be the same wall. Another coal miner in West Virginia claimed miners had found a WELL CONSTRUCTED CONCRETE BUILDING! "...the more you know."
10 More Unsolved Mysteries of the World - Listverse
Over the last few decades, miners in South Africa have been digging up mysterious metal spheres. 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 rock in which they where found is Precambrian – and dated to 2.8 billion years old!