Sea of Nurnen

The Sea of Núrnen, also called Lake Núrnen, was a large inland body of water in Mordor. It was roughly half the size of the Sea of Rhûn.
After the War of the Last Alliance Gondor built defensive outposts around Núrnen to watch over it and protect the lands around it from invading forces from the south trying to enter Mordor through the Nargil pass. The southern tributary of the Sea of Núrnen is also given a name, the Gurthrant river (borrowed from MERP). However, when the Great Plague struck from the east, the Gondorian forts were abandoned, allowing the Corsairs of Umbar to claim them as places from which to direct pirate raids into Ithilien and Khand. Over the time these Corsairs, separated from their evil lords in Umbar, settled down as a peaceful independent nation, naming themselves the Tribesmen of Núrn and starting to farm the fertile lands around Mordor's inland sea. However, when Sauron returned, he fought with the Tribesmen - even though they tried to defend against him they were trapped inside Mordor, so he enslaved them to make food for his growing armies.
Núrnen was located in the southeast of Mordor. It was fed by rivers flowing down from the outcroppings of the Ephel Dúath and Ash Mountains that separated the volcanic plateau of Gorgoroth from the fertile volcanic plains of Núrn, and its waters were described by Tolkien as "the dark, sad waters of Lake Núrnen". Its waters were likely used for irrigation purposes to water the fields of Nurn.
After the War of the Ring, King Elessar granted the lands around Núrnen to the freed slaves of Mordor.