In Generation I, the cave was inconsistently referred to as the Unknown Dungeon (Japanese: ななしのどうくつNameless Cave) in addition to its name. This moniker would later be reused for the Unknown Dungeon in Kalos, which is a direct reference to Cerulean Cave.
In Pokémon FireRed and LeafGreen, players must further prove themselves before entering the cave, which is done by completing the quest on Sevii Islands and restoring the Network Machine to operational status. More Pokémon can be found only by fishing, while wild Pokémon found just by walking around in the cave are reduced in number. Pokémon FireRed and LeafGreen also fixed the awkward Pokémon distribution from the Generation I games, which included several misplaced species such as Dodrio and Pokémon that are otherwise only obtainable by stone-induced evolution.
In Generation II, Cerulean Cave completely collapsed but the leftovers of Mewtwo's presence remain and are found in the lake near the cave, in the form of the item Berserk Gene. In Pokémon HeartGold and SoulSilver, Cerulean Cave has been rebuilt and Mewtwo can once again be found inside. Like in previous generations, an NPC stands in the way until the player has obtained all eight Kanto Gym Badges (as opposed to defeating the Pokémon League at Indigo Plateau, the requirement in the Generation I games and Pokémon FireRed and LeafGreen).
Geography
Encountering Mewtwo in the cave in FireRed and LeafGreen
Cerulean Cave is a large underground complex of maze-like tunnels which extend out from Cerulean City to the mountains beyond. With the dark entrance positioned on the outskirts of Cerulean City, only accessible by surfing down Route 24 to the entrance near the northern border of Cerulean City, the cave has been renowned for its mystery. The entrance to the cave looks like it has been carved out from a hillside, forming an indented entrance into the hill. An NPC guards the entrance to the cave and only allows Trainers who own all eight Kanto Gym Badges and have entered the Hall of Fame to enter.
At the deepest point of the cave lives Mewtwo, a Pokémon that can only be found in Cerulean Cave. Mewtwo was created by man in the Pokémon Mansion before it became too powerful and escaped, exiling itself to the cave, never to be seen again.
Cerulean Cave has had many different layouts over time, getting a change from not only one generation to the next but also from one game release to the next. Despite these changes, the first floor is mostly unaffected and has a large underground lake which covers most of the area. Using the complex of stairs and ladders, Trainers can make their way around the lake and the first floor of the cave. They will reach the upper floor, which has no underground river, but a like maze-like structure formed by large boulders and rocks that must be maneuvered around in order to reach the end. Once Trainers get through the maze formation, they are able to climb the ladders and get around the underground lake on the basement floor, which will lead them straight to a dead end and to Mewtwo.
Speleology
It is unknown how the cave was formed, but pictures of the cave suggest it is a solutional cave. A solutional cave is formed in the soluble rock, limestone, which is dissolved by natural acid in groundwater that seeps through the rock, forming the cave systems. Because of this, stalactites and stalagmites have formed in the cave while water rises from the floor and drips from the ceiling.
Cerulean Cave is not listed as a Pokédex area in the Generation I games. Therefore, any Pokémon that can only be found in Cerulean Cave is listed as "Area Unknown" by the Kanto Pokédex in these games.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
Layout
The layout of the cave has been tweaked several times in the past, particularly in Generation I.
In Pokémon HeartGold and SoulSilver, the layout of the first and second floors is based on the equivalent layout from Pokémon FireRed and LeafGreen, while the layout of the basement is based on the respective layout from Pokémon Yellow. Rock Smash boulders are also present and Rock Climb surfaces were added.
Cerulean Cave appeared in the Pokémon Origins episode File 4 - Charizard. It was first mentioned by Blue, who had went there after hearing rumors of an extremely strong Pokémon living in there. He intended to catch it but got badly injured while battling it when his Blastoise was sent flying and crashed on top of him. After he had told Professor Oak and Red about this, Red soon recalled the journals he had found at the Pokémon Mansion at Cinnabar Island and understood that the mysterious Pokémon was in fact Mewtwo, an artificial Pokémon created by enhanching the powers of Mew. Wanting to help Professor Oak to fill up the Pokédex, Red then headed to the Cerulean Cave, where he encountered Mewtwo. Being amazingly strong and durable, Mewtwo was capable of defeating all of Red's Pokémon, except for Charizard. At first, even Charizard was losing, and was sent into the waters of the cave, along with Red. However, Red's Mega Stones then activated, Mega Evolving Charizard into Mega Charizard X and making it able to defeat Mewtwo, allowing Red to catch it.
