Númenor is the great island kingdom that sinks in J.R.R. Tolkien’s legendarium.
Númenor was raised from the sea by the Valar for the Men who had aided the Elves during the wars against Morgoth. Its people became renowned for long life, seafaring, knowledge, and power. Over time, fear of death and resentment toward the immortal Elves intensified among many Númenóreans.
Sauron eventually corrupted King Ar-Pharazôn and encouraged the Númenóreans to challenge the Ban of the Valar. When Ar-Pharazôn sailed against the Undying Lands, Eru reshaped the world. Númenor was destroyed and drowned, while Elendil and other faithful survivors escaped to Middle-earth.
The destruction is sometimes called the Downfall of Númenor or Akallabêth. Númenor is not Valinor, the realm of the Valar, and it is not Beleriand, the western region destroyed during the First Age. Its survivors founded Arnor and Gondor.