Flying saucer? You could just as easily see a psilocybin mushroom depicted there, which would indicate a different but still interesting experience… 
Hubrecht, I thought the same thing when I looked at the picture!
Me too! (and a useless 20+)
And, this could expand into myriad historical possibilities of many such ancient (alien) carvings and pictographs around the planet.
On that topic, I suggest people listen to the new Forum Borealis interview with Dr. Haraldur Erlendsson called “What you should know about Psychedelics”, some very interesting information in there…
Both von Däniken and Sitchin found evidence in ancient art as evidence of UFO’s, or alien contact. Most of them are found in art considered to be sacred or religious in nature.
Could the original founders of the site be honouring the entheogen that led them to build at that site? I’ve heard that cathedrals, pyramids, temples etc are often built on places of great concentrations of energy/spiritual power call it what you will.
Perhaps “tripping” on psilocybin is what allowed them to “see” the energy, and they wished to leave a memento in the stonework.
We know that Europe had mushroom cults. How long they lasted and in what forms, I don’t know.
Fascinating topic! Thank you for mentioning it! Imo, this is an excellent 11-minute video that discusses this mystery.
