Argument often advanced by UFO buffs and tabloid writers as evidence of ancient contacts with superintelligent ET's: to wit, that since, say, the
Mysterious Tower of Foobar-Quux in Carcossa, BFE (a somewhat fulsome description of which follows, stressing the difficulty of building its like today) was built by Carcossans (a similarly poetic description of whom follows, stressing that these people today are only a step or two above flint-knapping), then ordinary human beings couldn't have built it. They must have had help, or at least a good set of building plans, from Our Green (athem, Grey) Friends in the Sky. How great were the Ancient Carcossans, therefore, that they were favored by our interstellar buddies, and how wonderful their civilization (or, for the Intellectual Progressive version, lack therof) must have been! The rest of the article, if it goes on, is devoted to what and how we must live to get similar help on problems today...etc....
While this sounds like high praise, let's examine this more carefully: what the author is saying is not that Carcossans were so great, but that they were so stupid. They couldn't possibly done something so complex as, say, precision stonecutting, or constructing scaffolding, so they must have had help. After all, the modern Carcossan, without the help of Western education, is illiterate and uses their fingers to count (male Carcossans, due to their inherent anatomical advantage, can count to eleven). This is somewhat like comparing apples and oranges: ask an average Frenchman to build a chateau, and they'll most likely to be at the least, nonplussed. Yet, even without backhoes, power tools, or pocket calculators, Versailles was built, with no help from anyone than an architect, a few hundred artisans, and a whole lot of nonunion manual laborers. Very likely, the Mystery Tower is the lone survivor of a lot of other towers (some of which may have been pulled down to build The Big One) and built by a crack team of experienced Carcossan tower-builders who didn't readily share trade secrets, and died off in an outbreak of measles.
Which points up another point in the argument: the structures involved are usually either too old or too out of the way for people to cross-check accounts. Even though the Duomo in Florence was built using techniques ahead of its time, no one doubts it was built by local Florentines, or at least, Italians: there are detailed building plans extant, biographies of some of the people involved, etc. (The same is true of the Step Pyramid of Saqqara: older by far and in some ways much more of an engineering marvel than the Pyramid of Giza, it loses its mystery somewhat by the biography of Imhotep and account of its building written on its outside wall.) Point out an artificial pile of dirt and rocks in the middle of nowhere, and it will seem like a miracle, if only because the people there aren't busy piling up dirt right now and/or have somewhat colorful folktales concerning its construction. (Translation: Beats us how or why it got there.)
Somewhat shopworn, this strand of urban legend still crops up (pun intended) in the Weekly World News, various Afrocentric-oriented materials or Fox TV during Sweeps Week.