Sea of Núrnen
From Tolkien Gateway
The Sea of Núrnen was an inland sea in Mordor.
It held bitter water not suitable for drinking, but the area around it, Nurn, was fertile enough (watered by a river system coming from the Ephel Dúath) to feed the entire armies of Sauron.
In The Atlas of Middle-earth, Karen Wynn Fonstad assumed that the Sea of Rhûn and Sea of Núrnen were the remnants of the inland Sea of Helcar. The atlas was however published before The Peoples of Middle-earth, where it was revealed that the Sea of Rhûn existed already in the First Age, as an apparently different body of water than the Sea of Helcar.
