I'm not talking about Mordor. I'm talking about the continent between Middle-earth and the Lands of the Sun. This continent existed throughout the Ages of Arda, until it seems to have vanished after the Downfall of Númenor.
The Dark Land is southeast of Middle-earth. The Silmarillion states that the Númenóreans visited it in the Second Age.
But the land of Aman and Eressëa of the Eldar were taken away and removed beyond the reach of Men forever. And Andor, the Land of Gift, Númenor of the Kings, Elenna of the Star of Eärendil, was utterly destroyed. For it was nigh to the east of the great rift, and its foundations were overturned, and it fell and went down into darkness, and is no more. And is not now upon Earth any place abiding where the memory of a time without evil is preserved. For Ilúvatar cast back the great sea west of Middle-earth, and the Empty Lands east of it, and new lands and new seas were made; and the world was diminished, for Valinor and Eressëa were taken from it into the realm of hidden things.
The Silmarillion, "Akallabêth"
This is the only thing I'm aware of that even hints at the answer. I would assume they were part of the "Empty Lands", destroyed and replaced with new continents.
