Keith was being too kind. Honest scientists say that we don't know how these structures were built, because all known and documented technologies of the time won't work (i.e. copper saws, ancient cranes). "Established" archaeologists claim feats of engineering not possible with known tools.

Of course it is possible that (for example) the Egyptians had secret tools and technology that they did NOT document and that were used to build the pyramids. That doesn't explain the water erosion around the Sphinx, the monoliths in Baalbek, Lebanon, Gobekli Tepe, the cities under the Sahara, newly discovered pyramids in Latin America, etc. etc.

There is a huge amount of evidence that there were advanced civilizations 10,000-12,000 years ago that disappeared. How advanced we don't know, because, as Keith points out, after that much time the only thing that will remain are large stone structures.