Ard-galen

In the fiction of J. R. R. Tolkien, Ard-galen ("Green Region"), later Anfauglith, was the wide green plain that lay north of the highlands of Dorthonion and south of Morgoth's fortress of Angband in the Iron Mountains, in the First Age.

Warning: Wikipedia contains spoilers

In the first days after the rising of the Sun, Ard-galen was a green plain with rich grass, reaching from Hithlum and the Ered Wethrin[?] in the west to the Ered Luin in the east, and rising into highlands of Dorthonion in the south. But the plain was laid waste by rivers of flame and poisonous gases in the Dagor Bragollach and renamed Anfauglith ("Gasping Dust").

The battle of Nirnaeth Arnoediad was fought in the plain, and the dead piled up in a hill in the midst of the plain, named Haudh-en-Ndengin, the Hill of Slain, by the Elves, and Haudh-en-Nirnaeth, the Hill of Tears, and grass grew again upon the hill and nowhere else in Anfauglith.

Like the other lands around it, Anfauglith sank beneath the waves after the War of Wrath at the end of the First Age.