The island kingdom destroyed in the Akallabêth, J.R.R. Tolkien’s account of the Second Age, was Númenor.
Númenor was created by the Valar for the Men who had aided the Elves during the wars against Morgoth. Its people became powerful seafarers, and the island stood in the Great Sea between Middle-earth and Aman.
Over time, Númenórean kings grew resentful of the Elves and of their own mortality. Sauron exploited that fear, eventually becoming influential at the court of King Ar-Pharazôn. When Ar-Pharazôn attempted to invade Aman, Eru Ilúvatar changed the shape of the world and destroyed Númenor.
The catastrophe is called the Downfall of Númenor, or Akallabêth. Elendil and his sons survived and founded the realms of Arnor and Gondor in Middle-earth, linking Númenor’s fall directly to the later history of The Lord of the Rings.