Which zone was flooded during the Cataclysm expansion?

Thousand Needles was the zone flooded during the Cataclysm expansion. The destruction caused by Deathwing dramatically reshaped the zone, filling its deep canyon with water and turning much of the landscape into a vast inland sea.

Before Cataclysm, Thousand Needles was a dry, vertically layered desert region in Kalimdor. Its defining features included towering rock spires, narrow canyon paths, and the Feralas border. After the world-changing events of Cataclysm, rising water covered the lower parts of the zone, and the area became known for boating and aquatic travel.

The flooding was part of a much broader redesign of the original World of Warcraft zones. Cataclysm altered terrain across Kalimdor and the Eastern Kingdoms rather than simply adding new quest areas. Darkshore, the Barrens, and Desolace also received major changes, which can make the options seem plausible, but Thousand Needles is the zone specifically associated with being flooded. The zone's new geography also changed questlines and made water-based movement central to exploration.