??? is an unnamed optional dungeon in the remakes of Final Fantasy III. Though unnamed in the game, the 20th Anniversary Ultimanias refer to this place as (鉄巨人の洞窟, Tetsukyōjin no Dōkutsu?, lit. Iron Giant's Cave).
To find this secret dungeon, the player must receive all four letters from the Four Old Men and Prince Alus. In the mobile, PSP and PC versions, getting the fourth letter requires having one character to level 99 in one of their jobs, and getting their Job Master Item from the Legendary Smith. The player must also have defeated all bosses in Eureka and opened about 80% of the treasure chests. Arc must be the party leader to receive any letter from Prince Alus after the first.
Provided the player has completed all three of the optional dungeons, defeating Odin in Saronia Catacombs, Leviathan in Lake Dohr, and Bahamut in Bahamut's Lair, they 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 water's surface. 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 here instead of the Crystal Tower.
The theme that plays is "Sunken Temple", also known as "Beneath the Horizon" in the original NES version.