I love reading about ancient and modern conspiracy theories but pretty much all of them are fun bullshit.
I love how ancient aliens came through the vastness of space on some sort of hyperdrive in presumably metal ships and then started practicing building stone block pyramids in Egypt, from the little shitty ones that are falling apart all the way up to the big ones. Or alternativly Atlantis was this world spanning civilisation which inexplicably built step pyramids in the Americas, smooth pyramids in Egypt and stone circles in Europe. The only reason why they are not claiming that aliens built the Cathedrals of Europe is because paper had been invented and there's a historical record of everything that happened in their construction.
All the UFOs were just secret government aircraft. It's not a coincidence that you had saucer shapes UFOs in the 40s and 50s when they had the operation paperclip scientists trying to make use of experimental nazi designs, cigar shaped UFOs in the 60s and 70s when they were sending those spyplanes over russia/china and triangle shaped UFOs in the 80s and 90s when they were testing stealth bombers. Oh and fuck all in the 00s now everyone has a personal camera (same with ghosts).