The fictional island kingdom destroyed before The Lord of the Rings in Tolkien’s mythology is Númenor.
Númenor was an island realm granted to Men who had aided the Elves during the First Age. Its people became renowned for seafaring, longevity, and power. Over time, however, fear of death and resentment toward the immortal Elves contributed to Númenor’s downfall.
Under King Ar-Pharazôn, Númenor challenged the power of the Valar. The island was destroyed and removed beneath the sea, while faithful survivors led by Elendil escaped and founded kingdoms in Middle-earth, including Arnor and Gondor.
Númenor is sometimes confused with Gondolin, an Elven city destroyed during the First Age, or with Atlantis because Tolkien’s account also describes a catastrophic submergence. Tolkien’s legendarium uses the name Atalantë for Númenor in one of its fictional linguistic traditions.