A name used of fallen Númenor after it was lost in the Downfall. During most of the Second Age, the island of Númenor had been the pre-eminent power of Men in the world. It was created by the Valar as a Gift for the Edain who had aided the Elves during the First Age, and through its early history its people maintained a reverence for the Valar and a friendship with the Elves.

As the centuries passed, however, the Kings of Númenor became cruel and greedy for wealth, ravaging Middle-earth for its resources. They remained overwhelmingly powerful, nonetheless, and indeed the last King went to war against Sauron and forced the Dark Lord's surrender. Taken as a hostage to Númenor, Sauron was able to subvert the King and his Council, ultimately persuading the Númenóreans to attempt an invasion of the Undying Lands where the Valar dwelt.

Númenor's attack on the West failed disastrously. Not only was the King, Ar-Pharazôn, lost with all his armies, but the land of Númenor itself was destroyed. In the cataclysmic event known as the Downfall of Númenor, the entire island was drowned beneath the Great Sea, and only a few survivors, led by Elendil, were able to escape to establish themselves in Middle-earth.

Númenor had always been known by many names, but after the Downfall it acquired more. Among them was Mar-nu-Falmar, from the Elvish for 'land beneath the waves' or 'home beneath the waves'. That name was used by the surviving Dúnedain in memory of their lost homeland, though in fact the island realm may not have been entirely overwhelmed by the Sea. In the centre of the island had been a tall mountain, the Meneltarma, on whose summit was a hallow to Eru Ilúvatar. Legend said that the hallowed peak had survived the Downfall, and remained as an island, the Isle of Meneltarma, a last remnant of Mar-nu-Falmar that still stood above the waves of the Sea.