Land of the Undying?

[Gwaihir the Windlord]

It used to be an island over the western sea. However after Ar-Phazron's rebellion it was taken off the face of the earth t be somewhere outside of middle earth.

It wasn't actually in Middle-Earth. Middle Earth is one continent on the face of the world of Arda -- our 'Earth'.
Aman, the main body of the Undying Lands, is the oher continent, and similarly sized. I think it's still in the world, only there is a sort of enchantment around it to prevent Men from entering.