"Once, these lands contained lush forests, fertile lands and giant rivers and lakes. But after the great Cataclysm, all of that was lost."
Trevyr Blackgard

The Cataclysm, also known as the Great Cataclysm, the Great Tragedy, or simply the Tragedy is a term used to describe the apocalyptic events that occurred about 12,000 years ago in the Borderlands. The Cataclysm was a series of volcanic eruptions, acid rains, dust storms, wildfires, drought, heat waves, and electronic storms that rendered the Borderlands temporarily uninhabitable and permanently dry and unable to contain water. The year in which the Cataclysm occurred is known as the Red Year.

After the First Men invaded Westeros, the Children of the Forest used their magic to break the Stormlands from the rest of the continent, as they had done previously to shatter the Arm of Dorne. Their magic did succeed, but not as how they planned. They changed the Borderlands completely, creating a series of natural disasters that dried out the region completely, eradicating the First Men from the Borderlands.