The Sea of Rhûn or Inland Sea was a large lake or sea in northern Middle-earth that lay east of Rhovanion on the western borders of Rhûn. The Kine of Araw were found in the fields of Rhûn near the Inland Sea.
Theories
Christopher Tolkien once questioned whether the Sea of Rhûn could "...be identified with the Sea of Helcar, vastly shrunken". Karen Wynn Fonstad adopted this position in making The Atlas of Middle-earth. However, in The Peoples of Middle-earth and The Nature of Middle-earth, which were published after Fonstad's Atlas, the Sea of Rhûn and its surrounding geographical landmarks exist as far back as the Years of the Trees of the First Age at the time of the Great Journey, hundreds of miles west of where the Elves awoke near the Sea of Helcar.
Geography
The Celduin flowed from the north-west into an arm of the sea. West of the Sea of Rhûn was the land of Dorwinion, and roughly 200 miles (300 km) to the south was the eastern end of the Ered Lithui.
A forest stood at the north-eastern shore of the sea, and near the south-western shore there were many hills. The south-eastern part of the Sea seems to be occupied by a small wooded island.
In the Years of the Trees during the time of the Great Journey, the Eldar reached the Sea of Rhûn after marching 450 miles west or northwest from Cuiviénen. The craft of ship-making practised by the Teleri reached new heights in their efforts to traverse the Sea of Rhûn on their westward journey.
In the First Age, the shores of the Inland Sea were populated by tribes of Men who were migrating to the West. The Lesser Folk arrived there first and dwelt at the feet of the nearby hills. The Greater Folk came later in the north-east woods near the shores. The Men crafted boats and could sail the sea, but they did not meet often, and their languages soon diverged before they resumed their journey to Beleriand.
In the early Third Age, the Kings of Gondor such as Rómendacil I campaigned to those lands, and Turambar expanded the kingdom to the East. By the time of King Hyarmendacil I, the Inland Sea formed one of the boundaries of Gondor.
Vorondil hunted the Kine of Araw near the shores of the Sea.
During the time of Rómendacil II, the Inland Sea was apparently within the borders of the power of Gondor.