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The Great Sundering, also known as simply the "Sundering" or the "Cataclysm" (not to be confused with the cataclysm caused by Deathwing), was a world event which reshaped Azeroth approximately 10,000 years ago at the end of the War of the Ancients. This was caused after Queen Azshara created a portal for Sargeras in the Well of Eternity and Malfurion Stormrage led his forces to destroy the Well. The destruction of the Well created an unstable vortex within the Well's turbulent depths. When those channeling the portal were attacked at the Eternal Palace, it threw the carefully crafted spellwork into chaos.
The unstable vortex within the Well's depths exploded and ignited a catastrophic chain of events that would sunder the world forever. The massive explosion rocked the temple to its foundations and sent massive quakes ripping through the tortured earth, as the surging Well of Eternity buckled in upon itself and collapsed.
The implosion of the Well created a vortex that began to consume the central landmass of Kalimdor, and the seas rushed in to fill the gaping wound left in the earth. The continent had been blasted apart into smaller continents.
Prior to the Sundering, there was only one known continent on Azeroth, referred to as Ancient 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, Pandaria, the Broken Isles, Kul Tiras, Zandalar, 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, a storm of chaotic energies, which continues to rage to this day.