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Submarine Explorer 1 inside the Seafloor Cavern
The Submarine Explorer 1 (Japanese: かいえん １ごう Kaien No. 1) is a submarine built at Stern's Shipyard in Slateport City.
In the games
Team MagmaR or Team AquaSE tried to steal the submarine's plans twice before the submarine could be built.
After the submarine is completed, Stern reveals that its purpose is to explore the Seafloor Cavern, an underwater cavern on Route 128. However, Team MagmaR or Team AquaSE end up stealing the finished product and taking it to the cavern themselves. The only time the submarine is visible during the game is when it is parked in the entrance of the Seafloor Cavern while the villainous team is inside. Inside the Seafloor Cavern, one of the grunts remarks that the submarine is "brutal as a ride" because of its small size. The submarine is not seen again after the player clears the villainous team from the Seafloor Cavern.
In the manga
In the Pokémon Adventures manga
The submarine (background) and its "heart"
In Pokémon Adventures, two parts of the Submarine Explorer 1 (named the Kaien 1 in the Chuang Yi translation) are featured: the submarine's "heart", manufactured by the Devon Corporation; and the submarine itself, designed by Captain Stern. Team Aqua steal the submarine's heart in VS. Ludicolo, which enables the submarine to travel deep underwater, while Team Magma steal the submarine itself in VS. Slugma II. This eventually leads to the two team leaders forming a pact to join forces temporarily in VS. Walrein, and heading to the Seafloor Cavern together with the complete submarine in VS. Whiscash. Later, in VS. Armaldo, the submarine breaks apart due to being sent deep underwater without its heart component, and the wreckage floats to the surface. This is the last that is seen of the submarine.
Ruby was in the submarine when Team Magma stole it. He escaped via an escape pod and ended up at the river on Route 119.