Arm of Dorne
The Arm of Dorne was a land bridge which once linked southeastern Westeros and southwestern Essos. In place of the land bridge is now the archipelago called the Stepstones, which is located in the narrow sea between the Broken Arm of Dorne and the Disputed Lands.
Around twelve thousand years before the Aegon's Conquest, the First Men came to Westeros from Essos by crossing the Arm of Dorne. When the children of the forest and the First Men first went to war, the old songs say greenseers of the children used the hammer of the waters to make the seas rise and sweep away the land, shattering the Arm, in a futile attempt to end the invasion of the First Men. Although the Breaking of the Arm was successful, it was too late, for the First Men in Westeros had already crossed and the wars went on until the Pact.
Archmaester Cassander argues in his Song of the Sea: How the Lands Were Severed instead suggests the "Song of the Sea", a slow rising of waters taking place over centuries caused by a series of long, hot summers and short, warm winters that melted the ice in the frozen lands north of the Shivering Sea.