Númenor was Tolkien’s great island kingdom that sank beneath the sea.
Númenor was created as a reward for the Men who had supported the Valar and the Elves during the First Age. Its people, the Dúnedain, were granted unusually long lives, and the island lay in the sea between Middle-earth and the Undying Lands. The kingdom became a major power under a succession of kings.
Over time, many Númenóreans grew jealous of the immortality associated with the Elvish lands. Sauron exploited that fear after being taken to Númenor as a prisoner. Under his influence, King Ar-Pharazôn attacked the Undying Lands, and Eru Ilúvatar changed the world, destroying Númenor in the Downfall.
Survivors founded Arnor and Gondor in Middle-earth. The story is chiefly told in The Silmarillion and is also connected with the background of The Lord of the Rings.