21w10a is the latest snapshot for the upcoming 1.17 Caves & Cliffs update and adds the lush caves biome.
This biome, found underground, is filled with various flora, such as moss, azalea bushes, grass, cave vines, and dripleaves.
You will be able to point out places in the world that contain lush caves underground by the azalea trees that generate above them, which contain flowery leaves.
Note that the lush caves does not currently generate in default worlds; the "Single Biome" world type must be selected with this biome to generate it.
The textures of all ores have been updates, and the missing deepslate variants of ores – redstone, emerald, and copper – have been added. Deepslate generates at the bottom half of the underground.
Three new blocks have been added: deepslate infested with silverfish, and cracked versions of deepslate bricks and tiles.
For the third week in a row, ore distribution has been tweaked, making emerald and lapis more common and copper, iron, gold and redstone less common.
Additionally, the game now uses OpenGL version 3.2 for rendering, enabling much-needed graphical overhauls.
These graphical changes necessitate updating the game's minimum specifications, which are listed on a support page.
For a visual guide to the changes in this snapshot, see my video below: