The Great Sea makes up the deep waters between the continents of Azeroth. Every single sea in the world connects to it. This sea is so vast players will die of fatigue before they 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 Undermine await travelers in the future.
Players were also introduced to Vashj'ir, which is located deep underneath the Great Sea to the west of the Eastern Kingdoms and is the location of the gateway to Abyssal Maw, the elemental plane of water.