You're likely thinking of the Whirl Islands, which are accessed by using the S.S. Aqua, a boat located in Olivine City.
The Whirl Islands are a challenging area filled with trainers and puzzles, and they have a unique mechanic where the land is constantly shifting. You can only reach certain areas during certain times of the day.
Let me know if you have any more questions about Pokémon Gold or the Whirl Islands. 😊