0x10c was a space sandbox game Notch and a few other Mojang people including Eldrone and Tobias Mollstam were working on around 2012. While a few small bits and pieces of it are available (the in-game emulated ship computer, the facebuilder character customization menu, and an extremely early barely functional leaked build) the actual game as it existed near the time of cancellation has not surfaced anywhere. (More info here https://minecraft.wiki/w/0x10c)
As seen in a twitch livestream by Notch (https://www.youtube.com/watch?v=FUqeH2FH1dE) many Mojang employees had access to a multiplayer build of the game with a fully modeled and textured spaceship environment with physics, light and sound, and FPS PVP mechanics. The usernames of these employees can be seen when Notch opens the player list on multiple occasions.