In “The Lord of the Rings,” the One Ring slipped from Isildur’s finger into the Great River Anduin. The event occurred near the Gladden Fields, shortly after the Last Alliance defeated Sauron at the end of the Second Age.
Isildur had cut the Ring from Sauron’s hand using the broken sword Narsil. Rather than destroy it, he claimed it as compensation for the deaths of his father, Elendil, and brother, Anárion. Years later, Orcs ambushed Isildur’s company. He put on the Ring to become invisible and escape, then swam across the Anduin. The Ring betrayed him by slipping off his finger, exposing him to Orc archers, who killed him.
The Ring remained beneath the river for nearly 2,500 years before Déagol, a Stoor Hobbit, found it while fishing. Déagol’s relative Sméagol murdered him for the Ring and eventually became Gollum. A common film-related mix-up is the precise defeat of Sauron: Peter Jackson’s films show Isildur cutting the Ring from Sauron during the battle, while Tolkien’s account says Sauron had already been overthrown when Isildur took it.