Gollum finds the One Ring in the cave beneath the Misty Mountains.
Gollum was originally Sméagol, a member of a river-dwelling people related to the Hobbits. On his birthday, he saw the Ring after his relative Déagol discovered it in the River Anduin. Sméagol killed Déagol to take the Ring and eventually became the isolated creature called Gollum.
The Ring’s influence drove Gollum into the Misty Mountains, where he lived for centuries beside an underground lake. In The Hobbit, Bilbo Baggins enters Gollum’s cave, finds the Ring after Gollum has lost it, and uses its invisibility to escape.
The wording matters because Bilbo is the character who finds the Ring in Gollum’s cave, but Gollum is the earlier finder of the Ring itself. Tolkien later revised the chapter ‘Riddles in the Dark’ to make its account consistent with the darker history revealed in The Lord of the Rings.