Ura Zelda (裏ゼルダ, literally "Another Zelda") was the Nintendo 64 Disk Drive expansion disk designed for The Legend of Zelda: Ocarina of Time. However, due to the failure of the 64DD as a platform, the game was never released on that platform.
During the early stages of development, two significantly different Ocarina of Time expansions were planned for the 64DD. One, titled Ura Zelda, would include alternate and more difficult dungeons (such as the Unicorn Fountain) and new enemies, while the other, Zelda Gaiden, would include a different overworld and an entirely new adventure. While Zelda Gaiden would eventually evolve into The Legend of Zelda: Majora's Mask, Ura Zelda was never released on the Nintendo 64 as a standalone game.
Ura Zelda was actually a patch for Ocarina of Time. The 64DD version was designed so that the original Ocarina of Time cartridge could be plugged into the main Nintendo 64 console while the Ura Zelda expansion disk would be plugged into the 64DD that sat underneath the console. Upon the insertion of the Ura Zelda expansion disk, the Ocarina of Time title screen would change slightly, adding a "DISK" icon to the right of the logo. The size of this patch was supposedly about 32-64 megabytes, and due to the smaller maximum size of Nintendo 64 cartridges, the game could not otherwise be released on a cartridge.
The Legend of Zelda: Ocarina of Time Master Quest
Eventually, due to repeated delays in production and development, Ura Zelda's release as a 64DD add-on was scrapped, even as the game was finishing production. As a result, there is no 64DD version of this game available to the general public. Due to public demand for the completed game, however, parts of Ura Zelda were released on the Nintendo GameCube as The Legend of Zelda: Ocarina of Time Master Quest on a bonus disc for those who pre-ordered The Legend of Zelda: The Wind Waker. The game was not truly ported in the technical sense but merely running under emulation, so there were some noticeable inaccuracies in the presentation of both Ocarina of Time and Master Quest as they appeared on this disc.
However, whether Master Quest is actually the end result of the Ura Zelda project or merely a quick level-edit to quiet demands and entice preorders is a matter of debate among fans. One significant piece of evidence against it being Ura Zelda is that ROM rips of the game show the game to be the same number of megabits in size as a standard Ocarina of Time ROM, whereas the original concept was to include content above and beyond what an N64 cartridge could possibly hold. It also did not contain any of the promised additional enemies. Furthermore, if one looks into the compiled versions of both Master Quest and the original Ocarina of Time, all 64DD reference has been removed from Master Quest. However, there is an N64 ROM of Master Quest playable on emulators.