Mountain Pass, California, on I-15 just south of the Nevada
state line has a VERY large deposit. The mine and mineral production facilities were closed a couple of years ago do to China undercutting them on the price. (Everything is in place to restart.)
The mine is supposed to be reopened under new ownership. Unfortunately the new owners are Chinese-led MP Mine Operations LLC.
My wife grew up there. Went by there 20 years ago the only thing left were empty streets with bare foundations and a school building.