Númenor is the fallen island kingdom described in J.R.R. Tolkien’s Akallabêth.
The island was raised from the sea by the Valar for the Edain, Men who had aided the Elves during the wars against Morgoth. Númenóreans were granted long lives and great power, but they were forbidden to sail to the Undying Lands in the west. Over time, fear of death and resentment of this restriction corrupted the kingdom.
Under King Ar-Pharazôn, Númenor challenged the power of the Valar. Sauron was taken to the island as a prisoner but became the king’s adviser and encouraged rebellion. The resulting catastrophe sank Númenor beneath the sea, an event known as the Downfall.
In Tolkien’s languages, the island has several names, including Númenor, Westernesse, and Anadûnê. Akallabêth is the account of its downfall in The Silmarillion, so the answer is the kingdom rather than the title of the catastrophe.