The Underworld Sea is a complex of subterranean channels under the Blackspine Mountains which give the Dark Elves alternative access to the oceans. They were discovered (by chance) by the Dark Lord of Hag Graef Kaledor Maglen in -2343. Their discovery enabled the Dark Elves to easily launch raids upon the abundantly rich eastern civilisations of Cathay and Nippon. The caverns connect Hag Graef to the Sea of Chaos at the eastern coast to the Boiling Sea(Broken Lands) by the western coast. Flash floods and cave-ins are an ever present threat in its labyrinthine tunnels.