Massive ancient structures like the Pyramids of Giza, Machu Picchu, and Stonehenge were built by human civilizations using ingenuity and engineering techniques. Archaeological evidence, such as tools and records, shows that humans created these buildings with ramps, sleds, and large workforces. These structures often had cultural, religious, or political significance, and ancient people used mathematics and astronomy to achieve their construction. While some suggest extraterrestrial involvement, there's no scientific evidence to support this theory, and human innovation explains their grandeur.