Cerulean Cave changes slightly depending on which Pokémon game is being played. In "FireRed" and "LeafGreen," the cave can only be entered once you get the National Dex and collect the Ruby and Sapphire Gems. Once the prerequisites are met, you can work your way through the rooms and catch Mewtwo.Continue Reading
Cerulean Cave is located on the edge of the city. You can reach the entrance on Route 24 by using surf. Once you enter the cave, you are in an open room.
The first and second floors of Cerulean Cave are set up like a maze, and it is different in every game generation. In "FireRed" and "LeafGreen" versions, the route you must take to Mewtwo is fairly direct, and you travel between sections of the two floors via ladders.
Mewtwo is always in the basement floor of Cerulean Cave. In "FireRed" and "LeafGreen," Mewtwo is sitting on a mountain in the southwest portion of the level. Use surf to reach it.
After encountering Mewtwo, walk back to the entrance of the cave. Unlike the other caves in the game, this cave only has one opening.