Welcome to Gaia! ::

You get it the key to Midgar City (Sector 5 Key) on disc 2 or 3; First talk to the man standing by the doors back at Midgar City. He won't appear until after you've gotten the Highwind and rescued Cloud from Mideel. Talk to him, then return to the Bone Village on the Northern Continent. Chat with the dude by the door and pick the first option, then the second or third option. While you could place diggers and hunt for the Key yourself, if you want to cheat, go over to the destroyed plane above the dinosaur's skull. See the one excavator near the bottom of the screen? Near him is a tiny piece of metal sticking outwards (the nose of the ruined plane); position yourself underneath it and press up so that your character is looking forward, then dig.

WOO HOO I got it! Actually i didn't find it by the plane. Go back to the Number 7 gate and you will find a man wandering about. Talk to him and he will inform you that he has lost the key to the gate.(I'm Sure you have already done that). The only clue you get from him is that he may have dropped it in an excavation site. What excavation sites do we know of? There’s only one, and that’s Bone Village, so that will be your next destination.

Once there, talk to the digger guy and tell him you are digging for “normal treasure”, then set the diggers on the top level of Bone Village and set off the blast. The diggers should point to the location of the “Key to sector 5”, and once they do, have them dig there. Now you can enter Midgar whenever you please

Is there anything worth doing in Midgar after you reach disc 3? I mean, what's the point of going back there?