In alternative history circles there's a frequent claim that the massive trilithon at the temple to Jupiter in Baalbek is evidence of a lost civilization or alien interference.
A less fantastic claim is that the stones were simply there from a previous temple structure.
Contrary to common usage of trilithon – forming a kind of doorway looking structure, usually free-standing – these stones are laid flat, in a row, integrated and forming part of the temple's foundation:
It is not known who commissioned or designed the temple, nor exactly when it was constructed. Work probably began around 16 BC and was nearly complete by about AD 60. […]
The original method of construction remains an archeological mystery.
While I definitely doubt the more fantastic claims, and I can find archeological writings that attribute the stones to the Romans and how they likely achieved it, I can't find anything that states how the stones were dated to the Roman period.