According to the Pokégear, map 21FF may be in Blackthorn City or Goldenrod City.
The equivalent glitch map freezes the game in the Japanese version and in English Pokémon Crystal. In English Pokémon Crystal the equivalent glitch map may belong to Mt. Silver (v1.0) or Special (v1.1) and play no music.
Accessing map 21FFMap 21FF can be accessed when entering a door with the codes 012144D0 and 01FF45D0 activated, but it cannot be explored without the following walk through walls codes: 0108A3CE, 0108A4CE, 0108A5CE, 0108A6CE. The map is long, but very narrow.
No music (track 00 according to memory address C1A9) plays in the map. The map is very narrow, only being two tiles thick but it is actually long. If the player steps out of bounds they may get a Glitch Dimension or actually be taken to another map (which has the Route 27 music playing). There are tiles in the map that force the player to move in a certain direction, including south and east.
When the room is first entered it will appear pitch-black except for its two tile wide path which is mainly white. 'Yellow dot pattern' tiles can be seen in the room.
The player can use Flash to brighten up the room. The room appears to use a tileset based on the Ruins of Alph (perhaps it is that tileset) as Unown markings can be seen.
Though Flash can be used to light the area up, like in a cave, neither Dig or Teleport can be used to escape from the map. Fly cannot be used to escape from the map either.
Even after using the above walk through walls code, the player cannot walk through certain positions on the map from all directions.
Article on the forums.