The recesses of the deepest underground are known as The Dark, a network of catacombs beneath the former Flatheadia Dungeon.

In 883 GUE, Lucille Flathead's father spirited her away to The Dark to be protected from the Curse of Megaboz. Here the Flathead seed continued to spread. Still fearing of their lives, several generations remainded hidden in The Dark, apart from the rest of the Empire. This was not, ultimately, a wonderful place to raise a child and many of its occupants grew dismissive of emotions. Lucy Flathead was born here, circa 989.

SOURCE(S): Zork Grand Inquisitor (official strategy guide)