The Sundering is a historic event in Azeroth, happening immediately after the implosion of the Well of Eternity and resulting in the shaping of the planet we know today.
In this point in time, Azeroth was a single supercontinent known as Kalimdor---meaning "land of eternal starlight" in Darnassian. Because the Well was the epicenter of the entire landmass, its implosion resulted in the sinking of entire cities---such as Suramar and Zin'Azshari---and the movement of the broken sections.
It was not until recent times that the continent of Pandaria was discovered. According to Lorewalker Cho, Emperor Shaohao had enveloped the land in dense mist as a means to protect it from the Sundering itself. While the continent was undoubtedly moved, it is unclear from what section of the Supercontinent it came from.