Encircling Sea

Arda and the Encircling Sea (somewhat conjectural)

In its original form, all the lands of Arda were surrounded by a vast dark sea that stretched away beyond sight in all directions. This was Ekkaia, also called the Outer Sea, in which Ulmo had his dwelling-place. Though the Encircling Sea was immense, it was not endless, and it came to an end at the Walls of Night.

The Encircling Sea was distinct from the Great Sea of Belegaer that lay between Aman and Middle-earth, though their waters flowed together in the far North of the world (and also, presumably, in the distant south).

After the Downfall of Númenor the world became Bent, so that Mortal mariners could sail entirely around it (somewhat confusingly, these seas of the round world are themselves sometimes called 'encircling seas'). What became of the primal Encircling Sea in this new cosmic structure is unclear. Most likely, it survived with Aman in their own Undying domain, still accessible to the Elves but lost to most Mortals.