Also known as The Great Sundering, this was a catastrophic event which reshaped the world at the end of the War of the Ancients as result of the destruction of the Well of Eternity.
Prior to the Great Sundering there was only one continent on Azeroth, known as Kalimdor. In the catastrophe almost eighty percent of the original land mass was laid waste, scattering the remains to form the major continents and scattered islands known today: Northrend, Kezan, the Eastern Kingdoms, and the remains of the original continent still referred to as Kalimdor.
The site of the Well of Eternity became a swirling vortex of power, known as the Maelstrom. Much of the area immediately surrounding the Well of Eternity was blasted onto the ocean floor when the well erupted.