But the land of Aman and Eressea of the Eldar were taken away and removed beyond the reach of Men forever. And Andor, the Land of Gift, Numenor of the Kings, Elenna of the Star of Earendil, 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 Iluvatar 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 Eressea were taken from it into the realm of hidden things.

The Silmarillion, Akallabeth

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.

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.