In Final Fantasy VII, the crashed plane accessible via submarine is the Gelnika. Its wreck lies underwater and can be explored after the party obtains a submarine, making it one of the game’s major optional locations.
The Gelnika was a Shinra aircraft that crashed into the ocean. Players can find it by piloting the submarine to a submerged wreck near the Gold Saucer area. The interior contains powerful enemies and valuable treasure, so it is considerably more dangerous than many ordinary exploration areas.
The wreck is especially important because it holds rare equipment, materia, and items, including rewards useful for the game’s final stages. It also contains the unique Conformer weapon for Yuffie and the Highwind manual, which unlocks Cid’s Level 4 Limit Break.
The name is often written as “Sunken Gelnika” to distinguish the wreck from the aircraft itself. It should not be confused with the Junon Submarine, which is a controllable vehicle, or the Sector 7 Reactor, an industrial facility destroyed early in the story.