|Please note that this glitch only exists in the Japanese versions of the game, or is otherwise a glitch from a Pokémon game which was only released in Japan.
Map F121 is a glitch map in the Japanese version of Pokémon Crystal. In Japanese Pokémon Gold and Silver, there is also a non-freezing equivalent glitch map.
It can be accessed by activating the GameShark codes 01F13ED0, 01213FD0 in a Pokémon Center, going up the stairs and back down, but it should also be possible to access the map with arbitrary code execution.
According to the Pokégear, the glitch map is on Johto's Route 39.
It plays the Team Rocket HQ music and may appear as a series of repeating glitchy water/buoy blocks. However, as soon as the player enters the glitch map the game may freeze and/or fade out to a white screen.