Skylands Mod

This mod is a port of the old skylands dimension that was experimented with in Minecraft beta 1.6. It could be unlocked through modifications of the game and then replaced the overworld dimension.

Minecraft Wiki: Sky dimension

This mod adds back the skylands as a seperate dimension. You start your game as normal but you can later travel to the skylands.

In order to get to the skylands dimension you have to find a magic bean in a dungeon chest.

Once you plant this bean, a giant bean stalk shoud grow and you will be able to climb up into the skylands.