Caves of the Forgotten
In the last years of Númenor, Ar-Pharazôn its last King was seduced by Sauron, and persuaded that he had the power to take the lands of Aman from the Valar themselves. He set out with a great fleet, and led an immense army into the Calacirya, ready to do battle for the Blessed Realm.
At that time Manwë the Elder King gave up his power over the World and called on Eru Ilúvatar, who unleased his power against the Númenóreans. Their fleets and even their island home were swept away, but the invaders of Aman were not destroyed. The hills fell on them, and they were imprisoned beneath the Earth in the Caves of the Forgotten, where they still await the ending of the World. When the day of the Last Battle comes, it is said, they will be released from their caves, though the part that they have to play in the Dagor Dagorath remains a mystery.