Sunken Gelnika
The Gelnika is a Shinra airship that has crashed in the ocean (hence, it is referred to as the Sunken Gelnika). It has been shot down by one of the Weapons and can be found at the bottom of the ocean.
The Sunken Gelnika is an optional area that you do not have to travel through during the story of the game. You can only access it after obtaining the Shinra Sub, which you can do by completing the Underwater Reactor portion of the game. Check out the Underwater Reactor and the Submarine Fight Walkthrough pages more details.
There are several items that you can obtain by going there, though, including:
Conformer
Double Cut Materia
Escort Guard
Hades Materia
Heaven’s Cloud
Highwind- Megalixir (x2)
Outsider
Spirit Lance
Check out the Sunken Gelnika side quest page using the link below:
The Sunken Gelnika
Side Quests
The sunken transport plane is found at the bottom of the ocean. It contains some of the game’s strongest enemies as well as a huge assortment of end-game items.
The map below shows the location of the Sunken Gelnika on the world map (which can only be accessed underwater):
