First Deep

From Tolkien Gateway

The First Deep was the level in Khazad-dûm immediately beneath the Great Gates on the eastern side of the Misty Mountains. After the battle in the Chamber of Mazarbul the members of the Fellowship fled down seven flights of stairs and traveled about a mile, arriving at the Second Hall on the First Deep. It was on this level that the Bridge of Khazad-dûm lay.[1]