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 or the 64DD 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.
Eventually, due to repeated delays in production and development, Ura Zelda's release as a 64DD game 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 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 pre-orders is a matter of debate among fans. One significant piece of evidence against it being Ura Zelda is that ROM rips of the game (taken from the Master Quest GameCube disc) show the game to be the same number of megabits in size as a standard Ocarina of Time ROM, even though the purpose of it being on the 64DD was so that it could include more content than an N64 cartridge could 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 references to the 64DD have been removed from Master Quest.
In mid-2013, a team of fans lead by environmental artist ZethN64 were working on a project which aimed at recreating what Ura Zelda might have been was in steady development until being spontaneously canceled by Zeth himself. Aptly titled "Legend of Zelda: Ocarina of Time URA", it was being developed for the Nintendo 64 using the same engine as Majora's Mask. The project showed a great amount of potential through screenshots, videos, and even a complete original soundtrack formerly available through the project's now defunct website, where it gained ample praise and support from fellow Zelda fans. The story would have been set in Hyrule during the events of Majora's Mask. Since Hyrule was without the Hero of Time (due to him being in Termina), another hero was chosen to rise and defend Hyrule in his stead. On August 20th, 2015, YouTube channel Hard4Games, which was a large supporter of the Ura Zelda project, uploaded a video showcasing a supposedly leaked test rom from the project. The reasons for Zeth's cancellation of the project aren't known in their entirety, but it is commonly believed to have been done in a fit of rage.
On August 9th, 2016, another Zelda fan known as Alex S on GBATemp.net had salvaged all of Zeth's project files and promised to assemble a team to finish and release the project, now known as "Another Restoration of Ura Zelda" or Project ARUZ for short.