Thousands of years ago, during the migration of the First Men to Westeros, the Arm of Dorne broke into the Stepstones, the Disputed Lands of Essos, and the Broken Arm of Dorne. Maesters are not in agreement as to its causes, however, or whether it occurred in a single night or over centuries.
Songs tells that the greenseers of the children of the forest, unable to stop the First Men, gathered at Moat Cailin or the Isle of Faces and called upon their old gods around twelve thousand years ago. Some stories claim the greenseers sacrificed captive men to weirwoods, while others state the children sacrificed their own young. The hammer of the waters broke and cracked the land, collapsing hills and mountains. The Arm of Dorne fell into the sea, leaving behind the Stepstones. The greenseers' action failed to halt the spread of humans in Westeros, however.
Instead of attributing the Breaking to magic, scholars such as Maester Yandel believe it was a natural event possibly caused by natural sinking of the land. In Song of the Sea: How the Lands Were Severed, Archmaester Cassander suggests that the land bridge did not break, but instead was overcome by rising waters caused by the melting of once-frozen lands.