According to Tolkien’s chronology, Númenor fell in the year 3319 of the Second Age. The island kingdom was destroyed after its king, Ar-Pharazôn, led an invasion against Valinor in an attempt to challenge the power of the Valar.
Númenor had been granted to the Edain after the First Age as a reward for their struggle against Morgoth. Over generations, its people became increasingly fearful of death. Their fear helped create resentment toward the immortal Elves and the Powers of the West.
Sauron exploited that resentment after being taken to Númenor as a prisoner. He became a powerful adviser and encouraged the king to attack the Undying Lands. Tolkien’s account says that Eru Ilúvatar changed the shape of the world, while Númenor sank beneath the sea.
The event is sometimes confused with the end of the Second Age itself, which Tolkien dates to 3441. Númenor’s fall happened earlier, in 3319, and its survivors helped establish realms in Middle-earth.