Ekkaia

Arda and Ekkaia (somewhat conjectural)

A name for the vast Outer Sea that encircled Arda, stretching on beyond sight until it reached the Walls of Night that separated the world from the Void beyond. It was across the dark waters of Ekkaia that the spirits of mortal Men set out on their final mysterious journey from the Halls of Mandos.

Ekkaia is a relatively late concept in Tolkien's cosmology, but was at least partly equivalent to the region earlier known as Vaiya, which enfolded the Earth forming a region of air above it and a tenuous dark ocean beneath, under which Ulmo had his halls of Ulmonan. In later years, Tolkien set to work to revise this cosmology considerably, but this project was left incomplete, and so the ultimate nature of Ekkaia was never established with certainty.