Minecraft wiki entry for Caves_%26_Cliffs_Prototype_Data_Pack, associated text: Deepslate blobs no longer generate above Y=0.
Diorite, granite, and andesite blobs are larger and more spherical, but less common.
Underground generation has been extended 64 blocks down to go along with the downward height expansion.
Deepslate makes up a majority of the expanded area, with the stone above gradually transitioning to it from Y=-1 to Y=-7 until it is completely replaced by deepslate at Y=-8.
Certain types of ores and structures still generate in the deepslate layer, as well as tuff.
Dirt, gravel, diorite, granite, and andesite do not generate in this layer.
Lava replaces air below Y=-53 instead of below Y=11.
The bedrock layer has been moved down to Y=-64.
Fossils are able to generate at any height instead of just between Y=40 and Y=49.
Fossils below Y=0 generate with deepslate diamond ore instead of coal ore.
Caves have been completely overhauled.
Now stretch down to Y=-59.
New noise caves, coming in three forms: cheese, spaghetti, and noodle.
Cheese caves have large stone columns stretching from the bottom of the cave to the top.
Very large, often large enough to fly through with elytra.
Abundant with springs, aquifers, and ores.
Spaghetti caves are long, thin caves that have small aquifers and are more similar to original caves.
Noodle caves are thinner, squigglier, and more claustrophobic variants of spaghetti caves.
Contain aquifers, bodies of water with different heights, independent of sea level (Y=63).
Only used below Y=31, meaning any noise caves between Y=31 and Y=63 are flooded with water.
Aquifers below Y=0 sometimes generate with lava instead of water.
The original carvers still exist with their generation slightly tweaked to add more variation and to better blend in with noise caves.
New crevice carvers, which are deep, long cracks in the ground that are thinner and straighter than canyons.
Ore veins, long and rare formations of ore and rock, have been introduced.