A 1909 newspaper story claiming an Egyptian-style cave city was found under the Grand Canyon, with no trace since.
Documented
The Arizona Gazette articles
On March 12 and April 5, 1909, the Gazette described G.E. Kincaid's discovery of a vast cave with mummies and artifacts, supposedly secured by the Smithsonian. The story fits the Mulhatton hoax template of the era and was reprinted as far as Utah. No artifacts, photographs, or expedition records exist.
Assessment: Documented: the newspapers exist; everything they describe does not.
Stories
The Smithsonian cover-up
Believers note the canyon's Egyptian and Hindu place names, the paper never running a retraction, and gaps in Smithsonian records, and conclude the finds were classified away. The absence of evidence becomes the evidence.
Assessment: Interesting: the place names came from a 19th-century expedition artist's whimsy; the record gap is real but proves nothing about mummies.
Interesting claims
A true cave behind a false story
A third reading: something was found, but the newspaper mangled it beyond recognition, and the hoax verdict now shields whatever it was. The story dies; the canyon keeps its side of the bargain.
Assessment: Interesting: unfalsifiable by construction, which is exactly why it survives.
Sources and further reading
Arizona Gazette, 'Explorations in Grand Canyon,' March 12 and April 5, 1909
Deseret Evening News reprint, 1909
Smithsonian Institution public statement denying any Kincaid or Jordan expedition
Worth asking
Is the strongest evidence for a hoax also the strongest evidence for a cover-up?