Castle Mountains National Monument protects 20,920 acres of mountainous desert in the eastern Mojave Desert of northeastern San Bernardino County, preserving part of the Castle Mountains near Lanfair Valley. Established on February 12, 2016, it became the only one of three Southern California monuments created that day to be managed by the National Park Service. Bordered on three sides by Mojave National Preserve, it forms part of a larger protected desert region. Its most unusual feature is that it surrounds but did not initially include the 8,340-acre Castle Mountain Mine Area, highlighting the ongoing tension between desert conservation and mineral development.