In order to find this secret dungeon the player must complete every Mognet sidequest first, including making the Ultima Weapon and receiving all four letters from the Four Old Men and Prince Alus. In the iOS version, getting the fourth letter requires having one character to level 99 of their job and getting their master card from the legendary smith. The player also must have defeated all bosses in the Eureka cave and opened about 80% of the treasure chests.
Provided the player has also completed all three of the optional dungeons of Odin Leviathan Bahamut, the player will receive the necessary mail from Prince Alus, and must use the Nautilus to access the secret area. The location is halfway between the floating continent and the Dalg continent to the east, and appears as a shadow on the surface of the water. The dungeon consists of only one floor, which is very similar to the final level of Doga's Grotto.
The dungeon features the three most powerful enemies in the game, as well as a new original superboss. To balance this, they are capable of dropping pieces of the Onion Equipment.
The "???" dungeon features the theme "Underwater Temple", also known as "Beneath the Horizon" in the original NES version.