2. Surviving ancient structures around the world (pyramids, fortifications, megaliths), 5,000-10,000 years old or even older could only be built using advanced technology. The hardest and heaviest stones were used and put together with the greatest precision. The high tech engineering and planning seems to have been so developed that we cannot even reproduce it today. And it was done in a time when people had not even invited writing.
3. Ancient cultures and monuments show a remarkable knowledge of astronomy and some incredible references to star systems. Some ancient structures look like they had some kind of navigation or air travel function in ancient times (like a landing strip or landmark). They are clearly visible from the air and are so large that they can be best understood from an airplane.
4. Archeological finds of ancient civilizations are sometimes out of place artifacts and images. These depict things that did not exist in ancient times, such as airplanes, astronauts and light bulbs.
5. There was a sudden leap in development of humans. From homo erectus, who did not change much for a very long time, suddenly highly developed civilizations like the Sumerians appeared. Legends of peoples all over the world speak of an unknown, highly developed civilization that existed even earlier but has been destroyed. It is unlikely that humans suddenly became smarter all by themselves.