Could there be a mysterious lost city underneath the Grand Canyon that can only be accessed through a cave opening "almost 1486 feet down a sheer canyon wall"? According to a feature article in the Arizona Gazette of April 5, 1909, there absolutely is such a place. The story goes that one G.E. Kincaid discovered the entrance while traveling the Colorado River by boat seeking minerals when he noticed a strange sedimentary formation and decided to investigate. Mysterious Universe's Brent Swancer explores this strange tale and can't prove that the the lost subterranean city doesn't exist, anyway! From Mysterious Universe:
Under the direction of a professor "S.A. Jordan," the Smithsonian Institute then allegedly launched a thorough exploration into the cave to find that that the main passageway penetrated nearly a mile underground, about 1480 feet below the surface, where it joined a massive chamber of some sort, from which branched off a series of tunnels "like the spokes of a wheel." Along these tunnels could be found an array of rooms filled with numerous artifacts and having obviously been once inhabited, as well as some sort of shrine[…]
Kincaid also describes granaries containing seeds, a large storehouse made from what appeared to be a type of very hard cement of unknown origin, a huge room, about 400 by 700 feet, that was thought to have probably been the main dining hall, and traces of a gray metal that could not be identified but which resembled platinum, as well as numerous yellow stones strewn about all over the floor. The entire cavern and tunnel system was expansive, estimated to have been able to house upwards of 50,000 people comfortably. Throughout the complex were found to be myriad carvings in the walls, doorways, and on artifacts that appeared to be some form of hieroglyphic[…]
The story has gone on to be discussed ever since, with many theorizing that it was the Egyptians, some forgotten lost civilization, or even reptilian humanoids, but there are plenty of skeptics that point out that it was likely a piece of fake news for a slow news day. For their part, the Smithsonian Institute has denied that such an expedition was ever made or that there is anything there, and they deny having ever had any association with a G.E. Kinkaid at all, who cannot be shown to have ever been a real person.
Of course, that's what They want us to believe.
"The Mysterious Tale of a Lost Subterranean City Beneath the Grand Canyon" (Mysterious Universe)