I recently loaded an old game on the third disc and tried to go to the underwater reactor so I can morph the Ghost ship for the book or whatever and I can't get in!!!! Will someone please tell me if you're not allowed to, or if there's a specific button to push or something because it's making me angry!

I havent played FFVII for a while.. so i'll try help

I think ur supposed to go to where u changed into Shinra uniform but along the way, if u remember there was a door originally unaccessible and there was a dog beside it.. go there and ur in.

If it did not work, go internet and check. Tons of guides out there.

You can fight a GHOST SHIP in the TUNNEL before you reach the reactor, so you don't actually need to go in at all. The tunnel made of glass that you can see all sea creatures through. Take the ELEVATOR in JUNON to get to it.
As far as I know, you can do this on Disk 3 as well as Disk 2.

OOOOOOH!! I was trying to take the sub into the Reactor! Thank you for clearing that up for me. I guess it makes sense and I just wasn't thinking.

sorry but I have to say this ... carry armor is a twat