Sea of Helcar
In the beginning of Arda, the Valar created the Two Lamps and two great towers on which to place them. Due to Melkor's deceit these were destroyed, and where Helcar, the northern tower, had stood a great inland sea was formed. This became the Sea of Helcar.
Christopher Tolkien and others have speculated that the Sea of Rhûn might "...be identified with the Sea of Helkar, vastly shrunken" (The War of the Jewels, pg. 174). In The Atlas of Middle-earth, Karen Wynn Fonstad assumed that the lands of Mordor, Khand, and Rhûn lay where the Sea of Helcar had been, and that the Sea of Rhûn and Sea of Núrnen were its remnants. In The Peoples of Middle-earth there are references to the Sea of Rhûn existing in the First Age, but no indication as to whether it should be equated with the Sea of Helcar or not.