In order to find this secret dungeon the player must get the Ultima Weapon and receive all four letters from the Four Old Men and Prince Alus. In the iOS/PSP versions, getting the fourth letter requires having one character to level 99 in one of their job and getting their Job Master Item from the Legendary Smith. The player also must have defeated all bosses in Eureka and opened about 80% of the treasure chests.
Provided the player has also completed all three of the optional dungeons; defeated Odin in Saronia Catacombs, defeated Leviathan in Lake Dohr, and defeated Bahamut in Bahamut's Lair. The player will receive the necessary mail from Prince Alus, and must use the Nautilus to access this 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 simple 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, making it easier for the player to farm for the Onion Knight's equipment in here instead of the Crystal Tower.
The "???" dungeon features the theme "Underwater Temple", also known as "Beneath the Horizon" in the original NES version.