What is the Maelstrom in World of Warcraft

The Maelstrom was created when the Well of Eternity collapsed in on itself during the Great Sundering.

There are rumors (unconfirmed at best) that in Cataclysm, Deathwing (the new big baddie) will emerge from Deepholm and enter Azeroth through the Maelstrom.

The Maelstrom is located dead center in the world map and depicted by a giant swirly vortex.