The theory that human civilization was jump-started by beings from elsewhere, and that our oldest texts are their flight logs.
Documented
Chariots of the Gods changed publishing, not archaeology
Erich von Daniken's 1968 bestseller assembled genuine puzzles: the scale of Baalbek's stones, the precision of Pumapunku, the Nazca geoglyphs visible only from the air, Maya ruler Pakal's sarcophagus lid posed like a pilot. His reading: non-human engineers, remembered as gods. The book sold tens of millions of copies and founded a television industry.
Assessment: The artifacts are real and the puzzles were, in 1968, less understood than now. Since then archaeology has produced conventional explanations for each: roller-and-ramp experiments, quarry marks, geological surveys. None of his specific claims has survived peer review, though the questions he popularized got better answers because he asked them loudly.
Interesting claims
Every religion is a misremembered contact report
The theory's deep form: Ezekiel's wheel, the Vimanas of the Mahabharata, the biblical 'chariots of fire', the Dogon's knowledge of Sirius B, all are technical descriptions filtered through pre-technical vocabularies. Von Daniken's successors run from Sitchin to the History Channel's Ancient Aliens, past 20 seasons, to the genre's current standard-bearer Praveen Mohan, whose temple videos carry the argument to hundreds of millions of viewers in a dozen languages.
Assessment: The Dogon-Sirius claim, the strongest card in the deck, collapsed under re-examination of the fieldwork. The method reads metaphor as blueprint wherever convenient and as metaphor wherever not. Filed as the 20th century's most successful modern mythology, and the assessment stands regardless of how entertaining the show is.
Stories
The cargo cults as mirror
The documented story the theory leans on: Pacific islanders, after wartime airstrips emptied, built bamboo runways and wooden headphones to summon the gods who brought cargo. If technology once looked like divinity to us, the ancient astronaut argument says, it could have happened before.
Assessment: The cargo cults are real and documented, the strongest analogy the theory owns. What they actually prove is that humans mythologize technology within living memory, which cuts both ways: it explains the ancient texts without needing any astronauts.
Sources and further reading
Ezekiel 1, the wheel-within-wheel vision
Genesis 6:1-4, the sons of God and the Nephilim
Erich von Däniken, Erinnerungen an die Zukunft (Chariots of the Gods?), Econ-Verlag, 1968
Worth asking
Von Daniken never answered the hard question: if the gods were astronauts, why does every ancient text describe them doing exactly what Iron Age humans imagined gods doing anyway?