It seems that proofs of aliens visiting Earth are often memorialized in ancient artifacts. For example, the construction of the pyramids, and the obvious alignments of portions of them with solar system objects. Also, cave paintings and writings supposedly showing aliens on Earth. See this example: Depiction of mythical Aztec universe discovered in natural pond near Mexico's Iztaccihuatl volcano. https://www.outerplaces.com/science/item/17446-ancient-aztec-map-universe-mexican-volcano I've always been amazed that aliens could travel hundreds, or maybe millions, of light years to Earth, interact with humans (or pre-homo sapiens), then either leave or die off, and not leave any better trace of their existences than some confusing scratching on cave walls, or ancient buildings like pyramids. Either: 1) Aliens never visited Earth, 2) aliens visited Earth but didn't want clear evidence left behind, or 3) who the hell knows. I believe aliens could have figured out how to leave trails. Vis. the 2001: A Space Odyssey monolith. In my wildest SciFi scenarios, a poor alien or aliens, lost a bet or the lottery, were chosen to fly to Earth, spread their DNA seed with monkeys and were then so disgusted they destroyed all evidence of the trip. Or the monkeys were so disgusted they killed off the aliens! This of course was about 3.5 billion years after they, or some other aliens, seeded the first DNA on Earth to start the process of life.