Sonic Gaiden is a 2003 fan-made hack of the original Sonic the Hedgehog game for the Sega Mega Drive. This hack is significant in the Sonic ROM-hacking community for several reasons. First, it's a hack of a Sonic 1 title; at the time of its release, such hacks were slightly rarer as Sonic 2 was the most frequently hacked Sonic game. Secondly, it is a hack of the Japanese Sonic 1 ROM; the U.S. Sonic 1 is more commonly used, but the Japanese revision has added features such as scrolling clouds and rippling water. Thirdly, the hack allows the option to play as either Sonic or as Mighty the Armadillo, who became popular as a founding member of the Chaotix crew and as a playable character in SegaSonic the Hedgehog. In addition, the hack includes new levels with new artwork and separate endings for each character.