In the beginning of Arda, the Valar created the Two Lamps. Due to Melkor's deceit these were destroyed, and where Helcar, the northern Lamp had stood a great inland sea was formed, which was named the Sea of Helkar after the tower on which the northern lamp had stood.
After the War of Wrath at the end of the First Age, the Sea of Helkar was drained through the Great Gulf and disappeared.
In The Atlas of Middle-earth, Karen Wynn Fonstad assumed that the lands of Mordor, Khand, and Rh鹡 lied where the Sea of Helcar had been, and that the Sea of Rh鹡 and Sea of N鷕nen were its remnants. The atlas was however published before The Peoples of Middle-earth, where it turned out that the Sea of Rh鹡 existed already in the First Age.
After the War of Wrath at the end of the First Age, the Sea of Helkar was drained through the Great Gulf and disappeared.
Christopher Tolkien and others have speculated that the Sea of Rh没n might "...be identified with the Sea of Helkar, vastly shrunken" (The War of the Jewels, pg.
In The Atlas of Middle-earth, Karen Wynn Fonstad assumed that the lands of Mordor, Khand, and Rh没n lay where the Sea of Helcar had been, and that the Sea of Rh没n and Sea of N煤rnen were its remnants.
During the War of Wrath that marked the end of the First Age, the Sea of Helcar was drained through the Great Gulf, and volcanic activity raised the plateaus of Mordor, Khand, and Rh没n from the former seabed.
The Sea of Rh没n and Mordor'sSea of N煤rnen were the only remnants of the inland sea.
In the north of Mordor during the War of the Ring were the great garrisons and forges of war, while surrounding the bitter inland Sea of N煤rnen to the south lay the vast fields tended for the provision of the armies by hordes of slaves brought in from lands to the east and south.