Lake Núrnen also known as the Sea of Núrnen was an inland sea in the middle of Nurn in Mordor.
The lake was fed by four unnamed major rivers, of which two flowed from the Ephel Dúath and two from a mountain spur that branched off from the Ered Lithui into Lake Núrnen
It was referred to as a bitter sea and held dark sad water, but the area around it was fertile enough to support great fields that were cultivated by slaves. An old meaning of sad is dark-coloured, in particular an unpleasant colour. Bitter is perhaps by analogy with sad or in the sense of unpalatable, referring to poisonous water from the rivers.
After the War of the Ring King Aragorn freed the slaves of Mordor and gave them ownership of all the lands around Lake Núrnen.
Commentary
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.
