After going to Crescent Island to look for the Espers, you learn the secret of Thamasa. Strago joins you and you head west to the Espers' Gathering Place.
Relm Sketch Bug
In version 1.0 of the U.S. Super Nintendo version of Final Fantasy VI, there is a bug that you might encounter when Relm uses her Sketch ability. This bug can have devastating effects on your game. The bug is not present in version 1.1 or later of the U.S. Super Nintendo version of Final Fantasy VI or in any of the non-SNES versions of the games (i.e. GBA, PlayStation, and mobile devices).
The bug is triggered if Relm tries to use Sketch, but misses. Even if you saved the game before the bug occurred, the game could still be affected even after you go back and reload the saved game. If you are unsure which version of the game you are playing, the safest way to avoid the bug is to not use Sketch.
Espers' Gathering Place
From Thamasa, go southwest until you are past the mountains, then go north to find the Espers' Gathering Place.
104. Insecare/Land Grillon
Adamanchyts/Adamchyts/Adamankarys are tough. Use the most powerful magic that you have.
In the first room, go northwest. Open the chest that you see there. Then go north and walk through the east wall to find a door. Go through it. You will be outside. Follow the straightforward path until you are in the cave again.
Go north up the small set of stairs, then go east behind the wall. Use the southeast exit. You will see golden statues there. Go south and check on the statues. (Note that the dialogue in the SNES version is a little confusing. The Espers made these golden statues, but the golden statues aren't the deities themselves. The deities are somewhere beyond the Sealed Gate, having turned themselves to stone.) After the cutscene, go south.
Ultros is weak to Fire and Lightning, but if you use magic attacks on Ultros, he will eventually glow red. From that point on, if you attack with magic, he will use a powerful counterattack.
Eventually, Relm joins the battle. Have her Sketch Ultros and he will be defeated. But if there is a chance that your game might have the Sketch bug that was mentioned at the top of this page, you can just defeat Ultros with regular attacks if you prefer.
After he is gone, make sure to equip Relm.
Use the northwest door and use the save point within. Walk onto the top square, then go through the north door and open the chest outside to get a Chocobo Suit. Then go back inside and go through the southwest door, and open the chest there to get a Tabby Suit.
Then go back inside and jump off of the southern broken bridge. Go east and south, and follow the path back to the room with the statues. Go through the northwest door to return to the room with the squares in the floor.
Step on the rightmost square, and go through the east door to find another chest. Then go back inside and jump off of the broken bridge. Go back to the room with the squares in the floor and use the bottom-left square. Go north and follow the straightforward path until a cutscene occurs.
Talk to Kefka to fight him. Use Shock.
Afterward, you will be able to use the Airship again. When you first get on the Airship, there will be a cutscene.
You need to go to the Floating Continent, but you should prepare yourselves before you go.
If you behaved correctly at the Emperor's banquet, then the Imperial soldiers pulled out of Doma and the Imperial Base. If so, be sure to visit Doma and the Imperial Base to get the treasures there if you have not yet done so. Be sure to check pots, clocks, and stoves there as well.
Be sure to explore any areas that you have not yet explored.
Learn New Skills
If you want to earn Magic Points/Magic AP, the Phantom Forest is a decent place to do that.
You can also take the time to teach Mog his dances. A good way to do that is to fly to the Returner's Hideout, then follow the path of Sabin's Scenario, and finally go through Mt. Kolts/Mt. Koltz to return to the Airship.
There are more Rages for Gau to learn now, so you might want to spend some time with him on the Veldt.
Go to the Auction House
You can fly to Jidoor (north of the opera house) and participate in the auctions to get the Espers Golem and Zoneseek/Zona Seeker. Unfortunately, there is only a 12.5% chance that the item in the auction will be an Esper, so you might have to try many times before either of the Espers show up. If someone else wins the Esper, you can still get it in subsequent auctions.
Go to the Floating Continent
When you are ready, press X while on the Airship, then check the controls and choose "Find the Floating Continent" to go there.