Arm of Dorne
From A Wiki of Ice and Fire
The Arm of Dorne was a land bridge. It linked Westeros and Essos together.
Around 12,000 years before Aegon's Landing, the First Men came to Westeros from the eastern continent by crossing the Arm of Dorne. When the First Men and the children of the forest went to war the greenseers of the children used their magic to shatter the Arm of Dorne, in a futile attempt to end the invasion of the First Men.
The shattered land bridge resulted in an archipelago named the Stepstones, which is located between the Broken Arm and the Disputed Lands.