Sounds more plausible than many preposterous elements of the story.
I’d have thought that Mr Emery C Kolb would have known about it and he didn’t say anything to me about it. He would have, if it had been true, I think. He lived at the top of Bright Angel Trail since 1904. Died at 90 something IIRC. He was a real character. Loved working for him.
Thanks Quix. Another problem with the story is, the purported discoverer is otherwise unknown — never had anyone by that name with the Smithsonian for example. Also, the off-limits areas are either too dangerous for hikers / canoeists and/or are environmentally sensitive. There are also no folkloric claims to any of this supposed stuff by any of the area tribes, and if this vast underground structure existed, it would be claimed, no doubt about it.
Funny thing about humans, they tend to settle in the same places over and over again, and tend to eschew places which are hard to get to, or hard to feed. So, for example, in the Phoenix area the remains of a canal system have been (mostly) destroyed by urban sprawl, but when European settlement began the valley was basically empty by comparison with whatever large settlement had been there (the one which built the canals, then disappeared). Not as much fun as Egyptians in the Grand Canyon, but probably more interesting. :’)
http://www.keelynet.com/unclass/canyon.txt