Quote Originally Posted by Cuchulainn
The key is in the ocean, you need to get the submarine & dive, it's in an underwater cave to the north of Junion. If you failed in the Submarine Minigame to sink the red sub, you need to go to Junion first to hijack the red sub, then get the key. North Corel is where you get the Sky-Tram to Gold Saucer, there's a reactor there.
Wrong key. What you're describing doesn't come into play until Disc 2. The Keystone is at the Gold Saucer, which is why BlackMagicShrine needs to get to North Corel.

The Final Fantasy VII section of EoFF provides two maps, one of which includes all the towns in the game. North Corel is on the same continent as Costa del Sol, the Gold Saucer, and Nibelheim. Hopefully, that gives you some actual reference in the game.