Arm of Dorne
From A Wiki of Ice and Fire
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 12,000 years before the War of Conquest, the First Men came to Westeros from Essos by crossing the Arm of Dorne. When the First Men and the children of the forest first went to war, the old songs say greenseers of the children used dark magics 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 shattering 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.