The belief that the Great Wall of China is the only man-made structure visible from space.
Documented
Astronauts cannot see it
The wall is long but only meters wide, built largely of stone matching the surrounding terrain. From low Earth orbit it is essentially invisible to the naked eye; from the Moon, no human structure is visible. Chinese astronaut Yang Liwei himself confirmed he could not see it.
Assessment: Documented. The claim predates spaceflight by two centuries; it was a guess that calcified into fact.
Stories
We want a monument the cosmos can see
The myth persists because it flatters: it says human effort registers at planetary scale, that building matters in the largest frame available.
Assessment: As an explanation of the myth's appeal, this is the honest layer. The wall is genuinely one of history's great constructions; it does not need the lie.
Interesting claims
Built to be seen from above
The fringe inversion: the wall was always meant for the sky, a message to watchers, gods, or returners, and the space myth is not an error but a recovered memory of intent. Ripley's Believe It or Not printed the from-the-moon claim in 1932, decades before orbit.
Assessment: The 1932 publication is documented, which proves the myth predates spaceflight and kills the conspiracy version cleanly. What remains is the wish: a civilization signing its name large enough for heaven to read.
Sources and further reading
William Stukeley letter, 1754, the Wall visible from the Moon
Ripley's Believe It or Not, 1932, repeating the Moon claim
Yang Liwei's post-flight statement, 2003: he could not see it (China Daily)
Worth asking
Why does a species that has actually been to space prefer the myth to the astronauts' testimony?