Vaiya, also called Ekkaia, Eccaia or the Encircling Sea, is the dark sea that surrounds the world of Arda before the cataclysm at the end of the Second Age. Vaiya flows completely around the world, forming a sea below it and a form of air above it. Arda is described as floating on Vaiya, like a ship on a sea.
Ulmo the Lord of Waters dwells in Vaiya, below the roots of Arda. Vaiya is described as extremely cold: where its waters meet the waters of Belegaer in the northwest of Middle-earth a chasm of ice is formed, the Helcaraxë. Vaiya cannot support any ships except the boats of Ulmo: the ships of the Númenóreans that tried to sail on it sank, drowning the sailors.
After Arda was made round Vaiya apparently disappeared, although it may have been changed into the upper atmosphere of the world.