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 when Queen Azshara created a portal for Sargeras in the Well of Eternity. The spell 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, 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, Azeroth was all one land mess. In the catastrophic explosion, eighty percent of the land mass was destroyed leaving behind the major continents and scattered islands that are known today: The Eastern Kingdoms, Kalimdor, Northrend, Pandaria, and the Broken Isles.
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. Deathwing was able to manipulate the Maelstrom during the Cataclysm to allow him to break free from Deepholm and ravage Azeroth.