The fictional island kingdom destroyed by the sea in J.R.R. Tolkien’s legendarium is Númenor.
Númenor is founded in the Second Age for the Men who aided the Elves during the wars against Morgoth. Its people receive exceptionally long lives and become powerful mariners. The island lies in the Great Sea between Middle-earth and Aman, the land of the Valar.
Over time, many Númenóreans become jealous of the immortal Elves and increasingly fear death. Their king, Ar-Pharazôn, eventually sails west with an army in an attempt to challenge the ban against reaching Aman. In Tolkien’s account, this rebellion leads to the island’s destruction and a major reshaping of the world.
Elendil and his sons escape the Downfall of Númenor and establish the realms of Arnor and Gondor in Middle-earth. Númenor is sometimes confused with Valinor, but Valinor is a region of Aman, not the island kingdom itself. Beleriand is another distinct land, destroyed much earlier at the end of the First Age.