Caves & Cliffs: Part II is in development at Mojang as the next major Minecraft update and you can now test the new world generation in a special version.

It's a major change in the way Minecraft is constructed, which is why Mojang decided to do it differently. In contrast to normal testing, this isn't an actual Snapshot that you download from the launcher. Instead, you should visit their blog to read instructions and a link to download the Snapshot.

It's possible for things to change but here's what's the difference:

New world heights and depths – New mountain biomes (lofty peak, snowcapped peaks and grove, meadow and snowy slopes). – New cave generation (cheese and spaghetti caves) and local water levels. Cave biomes are naturally created underground and inside mountains (lush caves and dripstone caves). – Natural variation of the shape of the terrain and elevation, independent from biomes. For example, deserts and forests could be created on hills without the need for a specific biome.