The Deep Sea Research Center is a two-part hidden dungeon. It's actually in plain sight, just located in an area of the map you're not likely to explore—the southwest corner. You can sail by with Balamb Garden, but you need to land the Ragnarok on top to enter.
The first section is a single room with a glowing core. The text that pops up is a hint: you will be attacked if you move while the core is glowing, even if you're using Diablos's Enc-None Ability. Wait for it to be completely dim before moving, and expect to go only a few steps each time. Your goal is the front of the core.
Once you reach the core, you'll be asked a series of questions. After each, you will have to fight a Ruby Dragon. The first answer is easy—there's only one option. The second question has two answers, and the one you're looking for is "Never." If you answer incorrectly, you will be sent outside and have to deal with the core again, and will start from the first question when you reach it.
The third question is tricky, because you need to choose the third of two options. Press down trice and you can choose the blank line. Get this one right and you'll face off against Bahamut.
To proceed further, you'll have to leave and re-enter the area. There will now be a save point at the top level. The next area is a puzzle. You'll have 20 units of steam to work with, and you'll need it to proceed from floor to floor. However, to find the ultimate prize, you'll need to have 10 units remaining at the bottom.
The layout here seems complicated, but it's actually fairly simple. Floors 3, 4, and 5 are each divided into three sectors, with 9 sectors in total. There are basically three paths—spending 4 RSP takes you to the expensive path (sectors 1, 4, and 7), while spending 2 takes you on the middle path (2, 5, and 8) and spending 1 takes you on the cheap path (3, 6, and 9). What makes things complicated is that you can't jump to a more expensive path.
Just getting to level 6 is easy—any path will do. You'll need 4 more RSP to open the door, which is achievable via all but the most expensive path. This will get you into the Deep Sea Deposit.
However, to reach the ultimate goal—Ultima Weapon—you'll need 10 RSP left when you reach the Deep Sea Deposit. The numbers don't add up, so you'll need a trick to manage this. There are two.
The simpler trick requires Zell to be in the party. Take the cheap path all the way, then have Zell handle the mechanism on level 6. This will leave you with 12 RSP in the Deep Sea Deposit—more than enough. However, using this trick comes with a steep price: you will be forced to fight a series of difficult battles on the way to Ultima Weapon.
The other option requires use of the steam room, accessible from sector 2. Spend 2 RSP on level 2 to reach sector 2, and another 4 RSP to reach the steam room. Inside you'll find a terminal that restores 7 RSP, which should leave you with 17. Take the cheapest option down to level 6, and you'll have exactly 10 RSP remaining when you enter the Deep Sea Deposit.
The first section is a single room with a glowing core. The text that pops up is a hint: you will be attacked if you move while the core is glowing, even if you're using Diablos's Enc-None Ability. Wait for it to be completely dim before moving, and expect to go only a few steps each time. Your goal is the front of the core.
Once you reach the core, you'll be asked a series of questions. After each, you will have to fight a Ruby Dragon. The first answer is easy—there's only one option. The second question has two answers, and the one you're looking for is "Never." If you answer incorrectly, you will be sent outside and have to deal with the core again, and will start from the first question when you reach it.
The third question is tricky, because you need to choose the third of two options. Press down trice and you can choose the blank line. Get this one right and you'll face off against Bahamut.
To proceed further, you'll have to leave and re-enter the area. There will now be a save point at the top level. The next area is a puzzle. You'll have 20 units of steam to work with, and you'll need it to proceed from floor to floor. However, to find the ultimate prize, you'll need to have 10 units remaining at the bottom.
The layout here seems complicated, but it's actually fairly simple. Floors 3, 4, and 5 are each divided into three sectors, with 9 sectors in total. There are basically three paths—spending 4 RSP takes you to the expensive path (sectors 1, 4, and 7), while spending 2 takes you on the middle path (2, 5, and 8) and spending 1 takes you on the cheap path (3, 6, and 9). What makes things complicated is that you can't jump to a more expensive path.
Just getting to level 6 is easy—any path will do. You'll need 4 more RSP to open the door, which is achievable via all but the most expensive path. This will get you into the Deep Sea Deposit.
However, to reach the ultimate goal—Ultima Weapon—you'll need 10 RSP left when you reach the Deep Sea Deposit. The numbers don't add up, so you'll need a trick to manage this. There are two.
The simpler trick requires Zell to be in the party. Take the cheap path all the way, then have Zell handle the mechanism on level 6. This will leave you with 12 RSP in the Deep Sea Deposit—more than enough. However, using this trick comes with a steep price: you will be forced to fight a series of difficult battles on the way to Ultima Weapon.
The other option requires use of the steam room, accessible from sector 2. Spend 2 RSP on level 2 to reach sector 2, and another 4 RSP to reach the steam room. Inside you'll find a terminal that restores 7 RSP, which should leave you with 17. Take the cheapest option down to level 6, and you'll have exactly 10 RSP remaining when you enter the Deep Sea Deposit.