Little Green Men
Although it sounds crazy, quite a few field researchers believe that aliens came to Earth and either constructed these spheres, or instructed people to construct these spheres. Dr. Sam Semir Osmanagich thinks that aliens visited and helped advance this human society farther than history has suggested, which is why they were able to build such insanely large structures. Some believe that the aliens that visited taught these humans how to build vehicles and levitation devices to move heavy stones.