You can get to the Mystery Zone without hacking, and it is in Gen IV games.
The Mystery Zone (Japanese: なぞのばしょ Mystery PlaceDPPt, ----HGSS) is an area in the Generation IV games that serves as "filler space" between normally accessible outdoor and indoor areas. It exists all around Sinnoh, to facilitate the precise entry into the Underground, which has tunnels that travel under areas where no routes or cities exist.
The Mystery Zone is normally inaccessible, but can be entered by means of tweaking and the surf glitch.
More information about tweaking
More information on the Surf glitch