Underwater Reactor, Huge Materia & Key to the Ancients (Disc 2) - FF7 Walkthrough
To get to the Underwater Reactor the first place you'll want to go is Junon. Speak with the guard by the elevator and pay him 10gil to ride up it then run down the street like we did the first time we were here. In the same area you see in the screen shot above you'll hear whistles and see some Shinra Guards doing practice posing.
Follow them to the Underwater Reactor entrance. The first entire half of this dungeon up to the boss is completely straight forward and has no treasures to be found. The only thing that's worth pointing out is the Ghost Ship enemy which you can Morph into a Guide Book item. This item is used in the Kalm Traveler Side Quest later in the game...
You can collect it now if you want or come back to this dungeon later and do it then. You can access the area the Ghost Ships are at any point later in the game but nothing further in the Underwater Reactor.
The room after the Save Point with the red lasers is where you will fight the boss of this dungeon - and boy is he difficult! He's similar in difficulty to the Demon's Gate boss that we faced off against in the Temple of the Ancients. You'll want to go all out against this guy using the Big Guard Enemy Skill right away and blasting away at the boss with your most powerful summons.
After defeating the boss there are three treasure chests for you to collect that are permanently missable if you leave here without collecting them. They're all pretty much in plain sight, use my screen shots below if you struggle with finding any of them.
Once you're on the submarine you'll have to do the submarine battle mini game. If you know what you're doing you can super cheese this event by speeding up immediately after starting and firing some torpedos at the red submarine. This will win you the submarine battle in less than 20 seconds.
If you fail to do this method you'll have to sail around looking for the red submarine and blow it up when you find it... The controls for the submarine are pretty straight forward:
Square: Fire Torpedo
Completion of this mini game will unlock it in the Gold Saucer Wonder Square. You can return there at any time to play this mini game again.
After taking out the red submarine Cloud will dock your sub at Junon and tell you to go someplace else first - ignore him and get back in the ship. There are two things you should collect right now from areas under the ocean, the first is the Huge Materia and the second is the Key to the Ancients. Screenshots of both these items are shown above.
In addition to these two items there are other things you can do in the ocean such as the Sunken Gelnika Side Quest or Lucrecia's Cave (Vincent's Ultimate Weapon & Limit Break). I've provided two different versions of the map below for you to use, the top one is the ingame map with each of the locations marked and the bottom one is a map I found on Google that is much more detailed.
I'd actually recommend you do the Sunken Gelnika right now before moving on as completing it will net you Yuffie's Ultimate Weapon the Conformer. Even if you don't use Yuffie there's reasons to get this weapon, such as it ignores the 1/8th damage rule with the Morph Materia. This will allow us to get some really OP equipment right now if you want it. For more information about this head over to my Cactus Island Guide.
Finally and last but not least, now that you have the Leviathan Scales Key Item you'll be able to complete the Da Chao Fire Cavern which is in Wutai.