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Beating the Elite Four in Pokémon FireRed & LeafGreen is a major milestone, but it’s just the beginning of your adventure. After defeating the Elite Four, you unlock access to the National Dex, which opens up a world of new Pokémon, including some of the most powerful Legendary and Mythical creatures.
One of these Legendaries is Mewtwo. You can catch Mewtwo in Cerulean Cave after you beat the Elite Four. However, before you can go after Mewtwo, there are a few steps to open the secret entrance in Cerulean Cave.
### How to Unlock Cerulean Cave
First, you need to defeat the Elite Four and have registered at least 60 Pokémon in your Pokédex. Once you’ve done that, restarting the game will trigger Oak to give you the National Dex, allowing you to catch more Pokémon.
After obtaining the National Dex, you’ll see that you also have the Aurora Ticket in your Key Items pocket. This ticket grants access to Birth Island, where Deoxys can be found, and the Mystic Ticket, which unlocks Navel Rock — home to Ho-Oh and Lugia.
### How to Get to Birth Island and Navel Rock
Start by returning to Island One of the Sevii Islands, where you’ll be asked to do a post-game quest for Celio. Celio needs a Ruby, which is located in Mt. Ember. From Island One, follow the path east, then north along Kindle Road until you reach Mt. Ember. Here, you will need to battle Team Rocket members and solve a series of puzzles involving boulders and Braille slabs to retrieve the Ruby. Once you have the Ruby, you can backtrack to find the Sapphire, which Celio also needs.
### How to Obtain the Sapphire
Follow the course to the east of the Pokémon Center on Five Island and fight your way through Rocket Grunts to reach the Rocket Warehouse. Inside, you’ll face a floor puzzle that involves arrows and navigation steps. The goal is to defeat the Rocket Grunts and the administrator to recover the Sapphire. Afterward, you’ll have to face the thief who stole it. Defeat him to bring the Sapphire back to Celio.
### How to Get the Ruby
Next, head to Icefall Cave near Cerulean City. The entrance is now open, accessible by surfing north of Nugget Bridge on Route 24. Inside Icefall Cave, navigate the icy paths carefully: you’ll need to use items like Max Repels, and teach Waterfall to a Pokémon to progress further. Cross the ice patches by following specific patterns and using ladders, eventually reaching the room with the Ruby.
### How to Find & Catch Mewtwo
Once you’ve opened Cerulean Cave, head to the lower section where Mewtwo resides. It appears at level 70, so bring a strong team—consider Pokémon like Snorlax, Gengar, Dragonite, or Golbat. Use status moves such as Sleep or Paralysis to weaken Mewtwo. Whittle down its HP and catch it with Ultra Balls. For shiny hunting, save right before the encounter; if it’s not shiny, reset and try again.
Mewtwo is one of the most formidable Pokémon to catch, so prepare accordingly and enjoy your new legendary addition.
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