The Inner Seas

 

"... the voyages of the Dunedain in those days went ever eastward and not westward, from the darkness of the North to the heats of the South, and beyond the South to the Nether Darkness; and they came even into the inner seas..." - The Silmarillion

There are, in fact, three 'inner seas' in the narratives of Middle Earth - two of which are on the LOTR maps (Sea of Rhun, Nurnen) and Helcar, an indeterminate distance in the east.

Nurnen we know the least about - lying within the boundaries of Mordor, the center of the region of Nurn, where slaves tended crops to feed Sauron's armies; after the war, King Elessar gave the land to the former slaves.  However, there were long periods of time when neither Sauron nor the Nazgul ruled in Mordor; fertile and well-watered (with four rivers feeding Nurnen), it likely was inhabited by Men for a long time - in fact, I would question whether it was considered part of "Mordor" in the days when the name referred to the darkness caused by the volcanic eruptions of Mount Doom.

Did the Numenoreans sail upon the seas of Nurnen?  It seems unlikely; Sauron was already well-established in Mordor by the time the Numenoreans were exploring Middle-Earth.  It is possible some explorers visited the shores, but the hostility of Sauron's forces, along with no easy access to the seas, would seem to disallow this.  However, ROTK does mention 'great roads' leading east and south to tributary lands, and early LOTR maps show a southern pass, the Nargil Pass.  

The Sea of Rhun has a much more detailed history, especially in the First Age.  Both elves and humans dwelt on its shores during their slow but steady drift westward, and the sea is probably legendary in Numenorean myths.  I've explored much of this already in my prior essay on the realm of Dorwinion (see), and I can only add that the realm may have had origins in remote Numenorean expeditions.

The Inland Sea of Helcar is not on the LOTR maps, and has been the source of speculation for decades; some have assumed, based upon a comment by Christopher Tolkien, that the Sea of Rhun was the 'remnant' of Helcar in the later ages, but this was later disproven when later volumes of the History of Middle-Earth mentioned the sojourns on the coasts of the sea in the First Age.  However, the most recent collection of Tolkien's unpublished writings, The Nature of Middle-Earth, would seem to indicate that Helcar is just off the map to the east, between the latitudes of Rhun and Mordor!  A calculation by JRRT in "The March of the Quendi" indicates that Cuivienen, the birthplace of elves, was only 250 miles eastward (and some distance south) of the Sea of Rhun... and Cuivienen laid on the eastern side of Helcar.  

It should be noted that the Orocarni - the Red Mountains - were immediately to the east of Helcar as well, running north-south; the range is likely one of the two eastern birthplaces of the dwarven clans. 

Considering the closeness of the Sea of Rhun and the vast open plains, the Sea of Helcar would also be likely to have been explored by the ancient Numenoreans.

Are there any other possible hints to other inner seas?  The Ambarkanta, Tolkien's pre-LOTR examination of the world, has Helcar formed by the destruction of the northern pillar by Melkor in the prehistory.  Looking at the (relatively) close geography of the three inland seas that we have just discussed, one can interpret that all three were created by the fall of the northern pillar.  Which, in turn, would imply that something similar happened to the southern pillar.  The Ambarkanta has at least one map that shows this, where it formed a connecting sea east and west, separating the traditional 'Middle-Earth' - or at least a portion a rough analogue to Africa, from a new continent called the Dark Land.  The South, sadly, is probably the least developed region of LOTR, so it is hard to say much about it, other than a decided lack of dwarves (apparently even the White Mountains was too far south for them to explore, prior to Gimli visiting Aglarond!)