The fictional land destroyed by the sea in J.R.R. Tolkien’s Second Age history is Númenor. The island kingdom is submerged during the cataclysm known as the Downfall of Númenor near the end of the Second Age.
Númenor was given to the Men who had aided the Elves during the War of Wrath at the end of the First Age. Its people became skilled mariners and built a powerful civilization, but their fear of death gradually turned into resentment toward the immortal Valar and the Elves. Under King Ar-Pharazôn, Númenor’s rulers challenged the divine order and sought immortality.
Sauron was captured by Ar-Pharazôn but corrupted the king and his court from within. When Ar-Pharazôn attempted to invade Aman, Eru Ilúvatar changed the shape of the world and sank Númenor. Elendil and his sons escaped, founding the realms of Arnor and Gondor in Middle-earth.