The Olmec writing in National Treasure: Book of Secrets leads to a revelation that Cibola, the legendary city of gold, exists. Ben Gates (Nicolas Cage) recounts the legend by reading it from a book:
BEN: "Later, when Esteban tried to find the city again, he never could. But the legend grew, and every explorer came to the New World in search of it."
BEN: "When General Custer’s search for gold ended with his last stand at Little Big Horn, it became clear none would ever find it."
Here's how the legend came about:
Quivira and Cíbola
The legend says that these cities grew very rich, mainly from gold and precious stones. This idea fueled many expeditions in search of the mythical cities during the following centuries.
Eventually, the legend behind these cities grew to such an extent that no one spoke solely of Quivira and Cíbola, but instead of seven magnificent cities made of gold, one for each of the seven bishops who had left Mérida.
For more on the subject, see The Best Indian Movies.