I saw something about this years ago on Unsolved Mysteries. A couple stumbled upon a huge underground river in Nevada or Ca when they were exploring caves and for some reason were scared to tell anyone where it was. I think the word got out and they were followed and harassed. Not sure what happened after that.
I liked that series.
Search “Kokoweef”. The legend tells of an underground river with shelves of gold. Eastern Mojave in California, between Baker and Las Vegas. Heck, you once, and maybe still can, buy shares in a mining company looking for it. I think I’ll pass, but ya never know...