The great island kingdom whose destruction J.R.R. Tolkien describes in The Silmarillion is Númenor.
Númenor was established in the Second Age as a reward for the Men who had aided the Elves during the wars against Morgoth. Its people became renowned for long life, maritime skill, learning, and power. The island lay in the Great Sea between Middle-earth and Aman.
Over time, Númenórean kings became increasingly jealous of the immortal Elves and fearful of death. Sauron exploited that fear, eventually corrupting King Ar-Pharazôn. The king led a fleet against the Undying Lands, and the island was destroyed in a world-changing cataclysm.
Númenor is sometimes confused with Atlantis because both are powerful islands destroyed by catastrophe. Tolkien’s account is distinct, however, and the survivors became the ancestors of the Dúnedain, including the Men of Gondor and Arnor.