Around 900 DR under Phandelver's Pact, clans of dwarves and gnomes began sharing riches within the Sword Mountains, then guided by "helpful" wizards descended from The Netheril, ores were extracted and refined to feed into a machine constructed under the direction of the enchanters. This became known as the Forge of Spells, and a great enterprise was formed crafting magic-imbued items.
In 951 DR an orc horde and dark forces (some say wizards, some say Illithid) sought control of the mines and the forge as part of a wave of invasions on the Sword Coast. In the battles, earthquakes sunk the cave beneath landslides - until discovered in 1480 DR by the dwarven Rockseekers. This led to the events of Lost Mines of Phandelver, including the great earthquake once more burying the place in 1491.
(See: wiki)