In April of 1909, the Arizona Gazette published a mildly amusing hoax about a Tibetan-style underground civilization found in the Grand Canyon, much like dozens of other hoaxes of its era, such as the 1885 Moberly, Mo. lost city hoax. The characters in the story never existed, and there is no evidence whatsoever that the cave in question ever existed.
Nevertheless, thanks to David Childress misunderstanding the article as referencing Egyptians rather than Tibetans (the subhead even reads "Orient"!), the hoax continues to live on as a staple of fringe history. I wrote about the hoax in my first ever piece of original online skeptical reporting, back in 2002, and that article was cited in Ked Feder's Encyclopedia of Dubious Archaeology. Below is the full text of the original newspaper hoax, preceded by an article from the same paper one month earlier that seems to have inspired the hoax, or was designed to set the stage for it.