The Smoking Sea is the body of water which separated the shattered islands of Valyria from the rest of the Valyrian Peninsula. The Smoking Sea was formed during the Doom of Valyria by earthquakes and volcanic eruptions. It is ill-omened and very few sailors would brave the Smoking Sea or sailing within sight of the Valyrian shore.
Four hundred years prior to the War of the Five Kings, the nation of Valyria was destroyed by a titanic volcanic cataclysm. Valyria was formerly a single peninsular, extending south into the Summer Sea. During the Doom the volcanic chain known as the Fourteen Fires erupted. The Valyrian Peninsula shattered, with the southern part becoming a cluster of islands. The sea that replaced the landmass became known as the Smoking Sea.
More recently, Euron Greyjoy has claimed that he successfully navigated the Smoking Sea to land on the island of Valyria look the former capital city of the empire.
The Smoking Sea divides the islands of Valyria from the mainland. It consists of one major channel extending from the Summer Sea to the Gulf of Grief and many smaller ones extending south, around the several major islands. The Smoking Sea is so-named because it frequently smokes, bubbles and boils. The sea sometimes gives off poisonous fumes into the air, fumes which can kill a man within moments. Steam frequently drifts across the surface of the water, leading to the claims that ghosts and demons haunt the sea.