The Copper Mountains are a mountain range that serves as the border between the kingdom of Fillory and the Wandering Desert.
Helping Sir Hotspots
In The Flying Forest, Rupert and Fiona Chatwin spend the novel searching for the source of a ticking sound that's keeping their friend Sir Hotspots, a Talking Leopard with exceptionally sharp ears, awake at night. The two later discover that a tribe of dwarves had hollowed out the Copper Mountains to transform them into an immense timekeeping device. In the end, the Chatwins enlist a friendly giant to bury the dwarves' clock deeper, satisfying both Sir Hotspots and the dwarf tribe.
Annexation of the Southern Nomads
While Eliot Waugh and Quentin Coldwater departed into the Eastern Ocean to search for the Seven Golden Keys, High Queen Janet Pluchinsky began a mission to annex the Wandering Desert and claim the territory for Fillory, bringing a battalion of troops over the Copper Mountains before stopping at the desert's edge. During the journey, Pluchinsky enlisted the services of a herd of Talking Elephants to carry her and her soldiers over the mountain. When an elephant fell off a cliff on the mountain, Pluchinsky watched as the rest of the herd revived the fallen elephant with magic, and later sent the entire battalion back while she continued on her mission solo.