The Japanese name 「ななしのどうくつ」 (Nameless Cave) is used in Generation I.
In the Japanese version of Pokémon Red, Green, Blue, and Yellow, this location is referred to by this name on the Town Map, although this was translated as "Cerulean Cave" in English.
In Japanese Pokémon Stadium (never released in English), as well as the Japanese version of Pokémon Stadium, this place name is found at the game data, although the name is not visible in-game because it is not selectable at the Pokédex map.
In Pokémon Red, Blue, and Yellow, this name is used by the NPC who blocks the cave entrance. This name is also used on the Town Map (only visible if the player uses the Town Map inside the Cerulean Cave), although the Japanese name used on the map is 「ななしのどうくつ」 (Nameless Cave) instead.
In Pokémon Gold, Silver, and Crystal, this location is referred to by this name in the game data, but because the location itself does not appear in the games, this name is not visible in-game.
The Poké Seer (available in Crystal only) is able to tell if a Pokémon was caught in this cave. However, this does not happen in normal gameplay because, in addition to the cave being unavailable in Generation II, any Pokémon transferred from Generation I games would not have this data.
In these games, this name is used by the NPC who blocks the cave entrance, and also in an email at the computer in Cinnabar Lab. In English, the email instead refers to "the cavern close to Cerulean" rather than naming it.
In the Japanese version of Pokémon Gold, Silver, and Crystal, this location is referred to by this name in the game data, but because the location itself does not appear in the games, this name is not visible in-game.
In Pokémon FireRed and LeafGreen, more Pokémon can be found only by fishing, while there are fewer different species of wild Pokémon found by walking around in the cave.
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. An NPC blocks the entrance 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). In Pokémon HeartGold and SoulSilver, if Mew is brought along, it will sense Mewtwo's presence and act apprehensive.
Encountering Mewtwo in the cave in Let's Go, Pikachu! and Let's Go, Eevee!
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 artificially created 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.
These items are hidden at certain spots of the cave and respawn occasionally or daily. All hidden items in Let's Go, Pikachu! and Let's Go, Eevee! may respawn either daily or more frequently, however at these locations in Cerulean Cave the item that respawns varies. The item received is not determined until it is picked up, so what the item is can be affected by resetting. However, if the player goes down a ladder and goes back up a ladder, the items and their spots will reset.
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. Breakable rocks are also present and Rock Climb surfaces were added.
Cerulean Cave appeared in File 4: Charizard. It was first mentioned by Blue, who had gone to the cave 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 enhancing the powers of Mew.
Wanting to help Professor Oak to complete the Pokédex, Red then headed to Cerulean Cave, where he encountered Mewtwo. Being amazingly strong and durable, Mewtwo was capable of defeating almost all of Red's Pokémon, excluding only his Charizard. At first, even Charizard was losing, and was sent into the waters of the cave, along with Red. However, Red's Key Stone and Charizard's Mega Stone then activated, Mega Evolving Charizard into Mega Charizard X and making it able to defeat Mewtwo, allowing Red to catch it.
Cerulean Cave appeared in The Discovery, where Trace and Green both entered it in order to find and catch Mewtwo. When Trace's Pidgeot was badly hurt by a swarm of Zubat and Golbat, he was forced to give up the race, allowing Green to reach the end of the cave first. There, however, she was upset to find out that Elaine had managed to catch Mewtwo first, and challenged her to a battle over it. However, despite using her MegaBlastoise, Green was still defeated when Elaine used Mewtwo against her. Green then handed Elaine Mewtwo's Mega Stones as a prize and jokingly suggested that she and Mewtwo should become her Pokémon, before leaving.
Cerulean Cave appeared in And Mewtwo Too?! and And Mewtwo... Three!. Upon hearing from Bill about a monster that had caused massive destruction in the northwest sector of Cerulean City, Red went to investigate Cerulean Cave, where the monster was said to have fled to. Once Red arrived at the cave, he and his Pokémon were soon caught in a tornado, which seemed to originate from inside the cave. Red and his Pokémon were saved from being blown away by Blaine and his Rapidash. Blaine explained that the tornado was actually Psywave, a Psychic-type move capable of both attacking and defending at the same time, and the user of the move was Mewtwo, the "beast" sighted in Cerulean City.
To Red's horror, Blaine revealed that he, while working as a scientist in Team Rocket, had been involved in Mewtwo's creation. Since Team Rocket had been unable to get enough of Mew's DNA to complete the Genetic Pokémon, Blaine had finished it by using cells from his own arm. In the process, some of Mewtwo's cells ended up in his arm, causing it to be horribly mutatated. This also gave both him and Mewtwo the ability to sense each other's presence. However, the mutated cells in his arm were slowly spreading to the rest of his body, and would eventually kill him. Realizing what kind of an abomination he had created, Blaine had subsequently left Team Rocket.
Blaine was determined to destroy the monster he himself had created, even if it would cost him his life. After putting Red inside of a protective fireball, he and Rapidash charged through the Psywave tornado at Mewtwo, creating a massive explosion. Despite the hit, Mewtwo was still capable of fighting. As Blaine was down for the count, Red tried to battle Mewtwo himself, sending out Poli. However, Mewtwo easily knocked the Tadpole Pokémon back with a giant spoon it created. Blaine explained that Mewtwo would form its Psywave into a tornado while fighting multiple enemies at once, and into a spoon for a single enemy. This gave Red an idea, and he sent out all of his Pokémon, causing Mewtwo to switch into its tornado tactic again. However, before the tornado got to full power, Red dived into the eye of the storm with Aero, and sent Pika diving further down into Mewtwo itself, capturing it with the Master Ball Blaine had given Red just moments earlier. Afterwards, Red gave Mewtwo to Blaine, telling him to teach it the kindness of humans, before heading off to the Indigo Plateau.
Cerulean Cave on the map
In the Generation I games, Cerulean Cave is only listed in the Town Map if the player uses this item while inside the cave. However, Cerulean Cave is not listed at all in the Pokémon location map from the Pokédex, likely to keep its existence as a secret until the player beats the game. For instance, Rhydon is only available at the Cerulean Cave in Pokémon Red and Blue, but Rhydon's Pokédex map simply says "Area unknown". Similarly, Golbat is available in multiple places including the Cerulean Cave, but only the other places are shown in the Pokédex map.