Underworld Sea

For hundreds of years, the Dark Elf raiding fleets were confined to the Sea of Chaos and the Great Ocean. Then they discovered the Underworld Sea, a huge underground water system that links the Sea of Chaos to Naggaroth's Western Coast.

The Underworld Sea is a treacherous place consisting of labyrinthine mazes of dark tunnels and strange caves. Movement is dangerous even on the well-known routes, for cave-ins and flash floods are a constant danger. There are also many strange and cruelly predatory creatures that inhabit this unearthly subterranean realm -- and their eyes are fare more accustomed to the gloom that those of the Dark Elves on whom they prey.

The most accomplished explorers of the Underworld Sea come from the Shade tribes, mountain-dwellers who forsook the life of the bleak cities in ages past. Yet even they have only uncovered a small fraction of the Underworld Sea's secrets. Each decade brings new discoveries, and rumours abound of an entire lost civilization hidden deep within the underground caverns.

Source

 * Warhammer Armies: Dark Elves (8th Edition) -- p. 13.