You can access the Sunken Gelnika in Part 2 of the original FFVII once you have obtained a submarine.
The Gelnika is a crashed airplane containing some of the most powerful gear in Final Fantasy VII.
There follows a list of the gear you can pick up here.
Double Cut (Command materia)
Hades (Summon materia)
Conformer (Yuffie's ultimate weapon)
Heaven's Cloud (powerful sword for Cloud)
Spirit Lance (strong weapon for Cid)
Outsider (very decent gun for Vincent)
Escort Guard (very powerful armor)
Highwind (Cid's best limit break)
Furthermore, if you first visit the Gelnika in Part 2 of the game rather than Part 3, you can fight Rude and Reno. You can steal a powerful Ziedrich armor from Rude and a rare Tough Ring accessory from Reno!
