The Caves of the Forgotten were a possibly fictitious cave complex believed to have been formed out of the hills that buried the Númenórean invaders of the Blessed Realm.
Seduced by Sauron, the King of Númenor, Ar-Pharazôn was persuaded that he had the power to take the lands of Aman from the Valar themselves. He set out with a great fleet, landed at the Blessed Realm and a host of the Númenóreans camped near the hill of Túna in the Calacirya of the Pelóri Mountains.
At that time Manwë, the Elder King, gave up his power over Arda and called on Ilúvatar, who then broke the world. Their fleets and even their island home were into an abyss that opened between Númenor and the Blessed Realm.
Meanwhile, Ar-Pharazôn and his mortal warriors were buried beneath falling hills. Yet, they are said to lie imprisoned within the Caves of the Forgotten, awaiting the day of the Last Battle when they will be released from their caves. However, the part that they have to play in the battle remains a mystery.