F-Zero GX was the GameCube's first and only F-Zero game when it launched in 2003. It continued the F-Zero legacy which began on SNES and has seen a few renditions since then. Alongside some home console and handheld variations, there was also the release of F-Zero AX in arcades. This version of the game was interesting primarily due to the hardware within the machine, which basically emulated the workings of a GameCube.
Modders have been peeking around in the code of many games, including F-Zero GX. Because of their curiosity we now know that F-Zero AX is actually found in its entirety within the other game's code. Even accessing the content is possible by simply using Action Replay codes.
F-Zero AX is basically GX but adds time-based races as would be expected of an arcade release. There are also a handful of exclusive racetracks as well. Unfortunately, this discovery doesn't grant all users access to the awesome sit down arcade experience that AX delivered.
Here is a video of the hidden game in action which was found by modder Rolf: