The central Mexican city of Teotihuacan was already in ruins when the Aztecs filtered down from the north and came across the massive pyramids that made up the city's center. When the Spanish conquered Mexico, the Aztecs informed them that the city was the "place of the gods," and they attributed its construction to their mythological deities.
Ancient astronaut theorists have seized upon this fact as "evidence" that Teotihuacan was built by space aliens, whom they take to be the originals of the gods. Erich von Daniken, for example, in Chariots of the Gods makes special note that the aliens/gods lived at Teotihuacan "even before homo sapiens existed" (p. 97). But as we have seen, it is common for later peoples to ascribe superhuman qualities to their predecessors. Additionally, it would be difficult to explain exactly how the aliens built three large pyramids and an avenue connecting them but not the vast city that surrounds them.
For a long time, it was thought that Teotihuacan was a "pyramid field," in von Daniken's words, because the houses were buried and invisible. But we know they are there now. The houses, elite and common, which make up Teotihuacan are less well-known than the pyramids, but they are built of the same materials in the same architectural styles. Did the aliens have an interest in constructing housing for 200,000 residents? If so, why did they choose to house most of the residents in small hovels without the benefits of alien technology, or even basic sanitation like running water? After all, the aliens were more generous to the roughly-contemporary Romans, who had running water. (Teotihuacan is believed to have been occupied between 100 and 700 CE.)
But here is the really interesting thing. Ancient astronaut theorists and alternative historians have made much hay out of the "mystery" of who built Teotihuacan, attributing the city to aliens, Atlanteans, a lost civilization, Phoenicians, and anyone other than native peoples. But as far back as the early nineteenth century, the native character of the place was well-known. Here is William H. Prescott in his monumental History of the Conquest of Mexico (1843):
But who were their builders? Was it the shadowy Olmecs, whose history, like that of the ancient Titans, is lost in the mists of fable? or, as commonly reported, the peaceful and industrious Toltecs, of whom all that we can glean rests on traditions hardly more secure? What has become of the races who built them? Did they remain on the soil, and mingle and become incorporated with the fierce Aztecs who succeeded them? Or did they pass on to the south, and find a wider field for the expansion of their civilization, as shown by the higher character of the architectural remains in the distant regions of Central America and Yucatan? It is all a mystery,—over which Time has thrown an impenetrable veil, that no mortal hand may raise. (vol. 2, p. 358)
Of course, on the other hand, Alexander von Humboldt, in his Political Essay on the Kingdom of New Spain (1811 English ed.) asserted that the pyramids of Teotihuacan and Cholula on the one hand and Giza and Dashur on the other shared an uncanny similarity in both size and geometry ("constructed on an analogous plan," he said), anticipating Graham Hancock's Fingerprints of the Gods by nearly two centuries:
[The pyramids' measurements] suffice also to prove the great analogy between these brick monuments erected by the most ancient inhabitants of Anahuac, the temple of Belus at Babylon, and the pyramids of Menschich-Dashour, near Sakhara in Egypt. (vol. 2, pp. 192-193, trans. John Black).
At least Humbolt assumed the builders were native Americans (the Toltec), immigrating to America from the "Mongol stock" of Asia (actually quite close to modern theories about the peopling of the Americas, if off by 10,000 or 20,000 years) rather than Aryans from Atlantis or space aliens.