I came across this information by chance via a whatsapp group I belong to, I looked into it and I came up with the following:
The story is that on the facade of the Salamanca Cathedral there is this engraving on the wall of an astronaut, supposedly this was found on the older of the two cathedrals in town, which was built in the 12th century. Well that is wrong, the astronaut is indeed engraved on a cathedral facade in Salamanca but not on this one, it is found in the facade of the newer, built in the 16th century, cathedral.
So you would think this about proves the existence of extraterrestrials or at least time travel because it is 400 or 500 years old, but no, unfortunately for fans of these theories the truth is the cathedral had a restoration in 1992 and someone decided to incorporate several objects like the astronaut, a gargoyle having an ice cream cone and a frog on top of a skull, because "it is a tradition of cathedral builders and restorers to include a contemporary symbol on the building as a means of "signing" their work."
So that is another hoax that has been viralized thanks to the internet.