Minecraft wiki entry for Development_versions, associated text: Completely overhauled.
Now stretch down to Y=-59.
The cavern now generates in 2 layers of different stone bases depending on the depth.
Cavern layer above Y=0 is made up of regular stone.
Stone transitions to deepslate from Y=0 to Y=-7.
Deepslate now completely replaces stone from Y=-8 to bedrock.
Certain types of ores and structures still generate in the deepslate layer, as well as tuff.
Dirt, diorite, granite and andesite do not generate in this layer.
Contain aquifers, bodies of water with different heights, independent of sea level (Y=63).
Aquifers below Y=0 sometimes generate with lava instead of water.[2]
Improved old caves, adding more variation.[3]
New noise caves, coming in 3 forms, cheese, spaghetti and noodle.
The original caves still exist with their generation slightly tweaked to better blend in with noise caves.
Cheese caves have large stone towers stretching from the bottom of the cave to the top.
Very large, often large enough to fly 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.