In J.R.R. Tolkien’s legendarium, the great island kingdom that sank beneath the sea is Númenor.
Númenor was an island realm established in the Second Age for the Men who had aided the Elves during the wars against Morgoth. Its people were granted long life and great wisdom, and the island stood in the western ocean between Middle-earth and the Undying Lands.
Over time, many Númenóreans became jealous of the immortal Elves and increasingly defied the restrictions placed on them. Under King Ar-Pharazôn, an invasion of Valinor brought about the island’s destruction. In Tolkien’s mythology, the world was changed and Númenor was overwhelmed by the sea. The survivors founded realms in Middle-earth, including Gondor and Arnor.
Númenor is often confused with Gondor because both are major kingdoms of Men and are connected by royal ancestry. Gondor survived in Middle-earth; Númenor was the island civilization destroyed before the main events of The Lord of the Rings.