The Great Sundering, also known as simply the Sundering or the Cataclysm, was a world event which reshaped Azeroth approximately 10,000 years ago at the end of the War of the Ancients. This was caused by the Well of Eternity imploding, due to it being too weak when Sargeras tried to enter through a portal fueled by the Well's energies. The implosion of the Well created a vortex that began to consume the central landmass of Kalimdor, and by the time it stopped, the continent had been split in three.
Prior to the Sundering, there was only one continent on Azeroth, referred to as Kalimdor. In the catastrophic explosion, eighty percent of the land mass was destroyed leaving behind the major continents and scattered islands that are known today: Northrend, Kezan, the Eastern Kingdoms, and the remnant still referred to as Kalimdor.
The site of the Well of Eternity became a swirling vortex of power known as the Maelstrom.
The concept of a sundering, or a world being reshaped, is a common theme found in fantasy. Tolkien described a reshaping of Arda, and lands lost to the seas in his writings, and the Dragonlance Dungeons and Dragons gameworld underwent the Cataclysm, where seas receded to the point that a coastal port town became landlocked. In Warhammer, the Gates of Old Ones once shattered and Chaos swept the world, damaging the land, primary on north and south poles. In David Eddings' Belgariad, the dark God Kal Torak attempted to use a magic orb that had been used to shape the world in anger and split the world in two and mutilated himself, an event similar to the Sundering.