In Pok?mon LeafGreen, the player must meet a number of requirements before being allowed access to the Cerulean Cave. Players must have entered the Hall of Fame, completed the Sevii Islands quest and gotten the Network Machine up and running again.
To enter the Hall of Fame, players must first defeat the Elite Four and the Pok?mon Champion. They are automatically entered once this occurs.
Players can begin the Sevii Islands quest prior to entering the Hall of Fame, but are not able to complete the quest until after they have defeated the Elite Four. The Sevii Islands quest requires players to visit seven islands and complete a number of different objectives, but the quest actually only needs to be completed up to the point of visiting Six Island before the Cerulean Cave can be accessed. The specific event that opens up the Cerulean Cave is when Team Rocket is defeated at the Rocket Warehouse on the Five Island. Along the way, players also repair the Network Machine on One Island.
Once the prerequisites have been met, use the Surf ability to navigate through the Cerulean Cave. The Rock Smash ability is also very useful to clear paths to bonus treasures and power-ups.