TSIL436 = The Secret in Locker 436
Found this gem on YouTube from a group called NAPAA — two hours long but worth it.
Description from YouTube: Underground cities talked about as evidence from many sources points to their existence. The Aztecs, Hopi, and other tribes point to their origins from the underground. In other parts of the world many underground cities exist, cultural similarities and legends from the orient, the record of Fusang talks of underground cities in a faraway land with a massive canyon. Other sources, from a Mexican treasure map to Spanish records also talk of massive underground caverns where treasure and records of an ancient people are contained. Native hieroglyphs also show evidence of underground cities and caves. Dan and Rick give the most in-depth account of evidence and clues that lend credibility to underground cities along the Grand Canyon.
More info on NAPAA is at http://www.tuscoro.com.