How Jungian of him. This, the “Hithlum passage,” from Morgoth’s Ring as well as Quenta Silmarillion:
But the cry of Morgoth in that hour was the greatest and most dreadful cry that was ever heard in the northern wold: the mountains shook, and the earth trembled, and rocks were riven asunder. Deep in forgotten places that cry was heard. Far beneath the halls of Angband, in vaults to which the Valar in the haste of their assault had not descended, the Balrogs lurked still, awaiting ever the return of their lord. Swiftly they arose, and they passed with winged speed over Hithlum, and the came to Lammoth as a tempest of fire.
There are other passages in QS that echo this reach into the depths. But I can’t remember them now.