Boiling Sea

On the west coast of Naggaroth lies a region of islands and monster-infested waters known as the Boiling Sea. It was created many millennia ago, before the recorded history of the Elves had begun. An immense earthquake ripped apart the coasts bordering the Far Sea and toppled them into the seas, fashioning the patchwork of islands that now run along the length of the western coast. The earthquake also tore deep rents in the sea bed, terrible wounds in the world's flesh that have never healed. Through these fissures, molten lava surges up from the world's core, heating the water and creating great bursts of explosive vapors. In parts, the sea literally boils, giving rise to the region's name.

Of all the Dark Elves' warships, only the Black Arks dare to enter this inhospitable and dangerous territory. They do so for one reason only -- to capture the monstrous sea creatures that have become such an important part of their fleet. Although many parts of the Boiling Sea are deadly to all forms of life, there are others where the mineral-rich waters teem with all manner of savage predators. Many such beasts, notably the ferocious Helldrakes and the vast Sea Dragons, have long served the Dark Elves as vessels of war -- at least since the Naggarothi learned how to sorcerously fuse battle-fortresses to the beasts' backs. With more and more of these creatures brought back from the Boiling Sea, the Dark Elf fleets have been dramatically transformed -- many Black Arks are now accompanied by flotillas of these primeval terrors.

Source

 * Warhammer Armies: Dark Elves (8th Edition) -- pg. 11