|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.|
It is a location which (according to the Pokégear) is in Cherrygrove City, with the Violet City/Olivine City theme playing.
If the player steps down enough via walk through walls, it may take them to an unknown other map in Pewter Gym with the Pokémon Gym theme playing (which may actually be Brock's gym).
If the player steps right enough via walk though walls, the game may freeze.
It is not possible to Fly from this location, Escape Rope, Teleport or Dig away; trapping the player if they save the game there.