The Great Sea makes up the deep waters between the continents of Azeroth. Every single sea in the world connects to it. This stretch of sea is so vast you will die of fatigue before you can swim any significant distance. Some say that somewhere within the Great Sea are lost islands still to be explored, and mysterious places such as the Maelstrom, the Broken Isles, and the Undermine await travelers in the future.
The Great Sea did not exist prior to the Great Sundering. After the Well of Eternity was destroyed and Kalimdor was split asunder, the waters that filled in the void became known as the Great Sea.
World of Warcraft: Cataclysm
This section concerns content related to Cataclysm.
The adventurers will also visit Vashj'ir, which is located deep underneath the Great Sea and is the location of the gateway to Abyssal Maw, the elemental plane of water.