[citation needed] Gold mining began at the site in 1936, but the rare earth deposits were not discovered until 1949 when prospectors in search of uranium noticed anomalously high radioactivity.

[12] In 1998, the mine's separation plant ceased production of refined rare-earth compounds; it continued to produce bastnäsite concentrate.

[16] In December 2010, Molycorp announced that it had secured all the environmental permits needed to build a new ore processing plant at the mine; construction would begin in January 2011, and was expected to be completed by the end of 2012.

The processing plant was in full production on June 25, 2015, when Molycorp filed for Chapter 11 bankruptcy with outstanding bonds in the amount of $US 1.4 billion.

[2] MP Materials is 51.8%-owned by US hedge funds JHL Capital Group (and its CEO James Litinsky) and QVT Financial LP, while Shenghe Resources, a partially state-owned enterprise of the Government of China, holds an 8.0% stake.

[21][22] In the 1980s, the company began piping wastewater up to 14 miles to evaporation ponds on or near Ivanpah Dry Lake, east of Interstate 15 near Nevada.

The scale is radioactive because of the presence of thorium and radium, which occur naturally in the rare-earth ore. A federal investigation later found that some 60 spills—some unreported—occurred between 1984 and 1998, when the pipeline and chemical processing at the mine were shut down.

After preparing a cleanup plan and completing an extensive environmental study, Unocal in 2004 won approval of a county permit that allowed the mine to operate for another 30 years.

Some outside China are concerned that because rare-earths are essential to some high-tech, renewable-energy, and defense-related technologies, the world should not be so reliant on a single supplier country[25][26] On September 22, 2010, China quietly enacted a ban on exports of rare-earths to Japan, a move suspected to be in retaliation for the Japanese arrest of a Chinese trawler captain in a territorial dispute.

"[27] The House Committee on Science and Technology scheduled on September 23, 2010, the review of a detailed bill to subsidize the revival of the American rare-earths industry, including the reopening of the Mountain Pass mine.

[29] According to Bloomberg, China in 2019 established a plan for restricting U.S. access to Chinese heavy rare earth elements, should the punitive step be deemed necessary.