The Cerulean Cave, also known as the Unknown Dungeon, is the final area in Kanto. It’s a three level cave that’s home to the toughest wild Pokemon in the entire game. Not only that, but the strongest Pokemon in existence, the one that was written about in the Pokemon Mansion, can be found in the basement of this dungeon. Bring the Master Ball with you to catch it without any effort!
This dungeon can be found by going north of Cerulean City to Route 24, then surfing southwest towards Route 4. This cave can only be accessed after becoming the Pokemon League Champion.
All Pokemon found while fishing will be the same on each floor. However, the Pokemon encountered while walking will vary depending on the floor.
The majority of this dungeon will be going back and forth between the first and second floors collecting items. There aren’t any Technical Machines here, so if you want to go straight to the final Legendary Pokemon, go all the way to the ladder at the southwest side of the first floor.
If you want to collect each item, you can explore the ground floor and collect three items: Max Elixir, Nugget, and Full Restore. To get the items on the second floor, you’ll have to take each ladder and navigate through a small maze.
The first ladder near the entrance will position you on the second floor to find a PP Up to the southeast. After finding the item, you can take the ladder to the north to return to the first floor.
You should now be on the north side of the first floor. Surf west to find a Max Elixir between some rocks near the river. Continue west and walk up the next hill to find a ladder back to the second floor. Navigate around the maze going eastward towards a Full Restore. The ladder back up to the first floor is further east.
Walk down the hill to the south, travel on foot to the west, and then climb up the hill to the north to find another ladder up to the second floor. The maze here will be a bit more difficult. Travel east, followed by south, and finally west to a fork in the road. The path to the north will lead to an Ultra Ball, and the path to the south will take you to a ladder back to the first floor.
Now that you’ve returned to the first floor, take the final ladder around the rocks to the south. This will lead you to the basement.
A Max Revive can be found up another hill to the east. After grabbing the item, follow the path to the southwest corner of the floor, then walk up another hill to the east to grab the last item of the game: an Ultra Ball.
Surf and hike your way to the southeast corner of the basement to find a legendary Pokemon. Make sure to save before interacting with it.
Legendary Pokemon: Mewtwo
Once again, Mewtwo is the strongest Pokemon in Red, Green, Blue, and Yellow! It’s even stronger than the Pokemon it was born / cloned from: Mew. If you still have the Master Ball from Silph Co, use it now to catch this monster without breaking a sweat. However, if you lost or already used the Master Ball, you’ll have to rely on Ultra Balls. Reduce Mewtwo’s health to the lowest possible percentage and inflicting the sleep status condition to increase your chances of capture.