You can only get into Cerulean Cave after you become the Pokémon League Champion and after delivering the Ruby and Sapphire to Celio in the Sevii Islands.
After delivering the gemstones to Celio, the person blocking the Cerulean Cave entrance in Cerulean City will disappear and you’ll be able to head inside.
Cerulean Cave is basically the final end-game challenge, so the prerequisites for reaching it are pretty steep.
Before you can enter Cerulean Cave, you’ll need to:
Surf is necessary to reach the cave and get to the end of it (where Mewtwo is located) but the most important prerequisites on this list are defeating the Elite Four and delivering the gemstones to Celio.
Until you finish both of the prerequisites, the entrance to the cave will be blocked by an NPC.
Where is Cerulean Cave?
Cerulean Cave is on the west side of Cerulean City.
However, the cave can’t be reached by simply walking.
First, head to the north side of Cerulean City, where the yellow nugget bridge is located. Walk north across this bridge onto Route 24.
Once you reach the end of the bridge, use Surf on the small stream of water just left of the bridge.
Follow this water path south until you come across the entrance to Cerulean Cave, which will be located on the left side of the screen (west of Cerulean City.)
Accessing Cerulean Cave
If you try to enter Cerulean Cave before becoming the Pokémon League Champion and delivering the gemstones to Celio, then a guard will block the cave entrance and will not allow you to pass through.
Defeating the Elite Four happens naturally throughout the Pokémon FireRed and LeafGreen story, so you’ll do that anyways in your playthrough.
But to start the gemstone questline, you need to head to One Island and talk to Celio.
As soon as you deliver the Sapphire to Celio on One Island, you’ll be able to head back to Cerulean City and enter the cave.
What’s Inside Cerulean Cave?
Since Cerulean Cave is the biggest end-game challenge in Pokémon FireRed and LeafGreen, it’s no surprise that it’s full of goodies.
The first most sought after prize in Cerulean Cave is the Legendary Pokémon Mewtwo, who is waiting at the end.
Other than Mewtwo though, there are many items to collect throughout Cerulean Cave!
|Cerulean Cave Items|
|Ultra Ball||1F||Hidden two tiles to the right of the ladder near the Nugget|
|Ultra Ball||2F||Southeast area (accessible from the eastern ladder on 1F, requires Rock Smash.)|
|Ultra Ball||B1F||Northeast section of the floor|
|Full Restore||1F||Southwest corner of the floor|
|Full Restore||2F||East side of the floor, accessible from the southwest ladder on 1F (requires Rock Smash)|
|PP Up||2F||Southwest corner of the floor accessible from southwest ladder on 1F (requires Rock Smash.)|
|Max Revive||B1F||Northeast corner of floor|
As you can see, Rock Smash is basically a hard requirement for getting all of the goodies in Cerulean Cave. So if you don’t have it yet I recommend making your way to Kindle Road!