Underwater Submarine Fight
You have the option of either fighting the Shinra Soldiers or taking them prisoner after you enter the bridge. It doesn’t matter which you choose. Examine the pilot’s chair to begin. You can either read the instructions or jump in the seat.
Controlling the submarine is fairly straightforward:
- Use the
button to speed up - Use the
button to fire a torpedo - Use the
button to slow down
The goal is to locate and destroy the Red Submarine, which contains the Huge Materia inside it.
You can end this fight quickly if you are speedy enough. The Red Submarine starts just a short distance in front of the sub you’re in.
Quickly speed up, lock on the target, and fire torpedoes with the button, and the mission is complete! Check out the video below to see this in action:
The game isn’t too challenging if you have to hunt down the other submarine, either. You will now receive the Submarine vehicle, which will allow you to access many new areas around the world map.
The game will guide you where to head next, which is back to the “Junon Airport.”
Underwater World
The game will automatically resurface the submarine and park it in the dock next to Junon. However, there are several things you can and should do now that you can explore areas with the submarine. Hop back into the submarine and press 
to go underwater.
Emerald Weapon
Warning
Do not venture too close to the giant floating monster (Emerald Weapon) that you can see underwater. This boss is far too powerful for you to defeat at this point in the game. Each time you submerge your submarine, Emerald Weapon will appear in a different location.
Red Submarine Wreckage - Huge Materia
The first thing you should do is find the wreckage of the red submarine that you destroyed during the submarine battle. The red submarine is on the seabed, at the bottom of the ocean, in the southernmost area of the sea.
Use the map below as well as the screenshot below to help you locate it.
Bahamutype-0
Achievement/Trophy
To obtain the 
Bahamut ZERO Materia later on in the game, which will reward you with the 
Bahamutype-0 Achievement/Trophy as well, you need to get a few items, including each of the four Huge Materia with this one being the third out of the four.
You should restart your game if you were unsuccessful in tracking down the red submarine, as there is no way to obtain this Achievement/Trophy unless you retrieve this Huge Materia.
Key to the Ancients
Travel through the crevasses to the northernmost section of the ocean. Turn right at the very end of the pathway to see a small, hidden cave that leads to the Key to the Ancients. Touch the Key to pick it up. Use the map and the screenshot below to help you find it.
You need the Key to the Ancients for a section of the game that will be coming up shortly.
There are several other side quests that you can complete before proceeding to Rocket Town. These side quests are listed on the next page.