During the intial playthrough of the main story of Final Fantasy VII, you'll heard about a Shinra plane crashing, but you won't be able to even locate the Glenika until your at Disc Three. Once you recieve the submarine, you can locate the crashed Glenika. The location is somewhat near Junon. Inside the Glenika, there are some high-level monsters you can encounter, and there are rare items you can locate inside, that you shouldn't miss. To name a few, Yuffie's ultimate weapon, the Conformer, "Double Cut" Materia, "Hades" Summon Materia, and Cid Highwind's Level Four Limit Break Manual, Highwind. Sometimes when you try to leave, you may encounter Emerald Weapon, if you don't wish to fight him, then return to the Glenika, and then re-escape the Glenika, and the WEAPON should be gone. If your looking for a good place to train, the Crashed Glenika is also one of the only good spots to train, even at a high level.