Caverns
Caverns are reservoirs of liquid (water, lava, or acid) found underground in the Overworld and in the Nether.
Generation
Caverns are structures that generate underground in the Overworld, and randomly in the Nether (as the nether has no main ground level). They will often generate close to caves and/or dungeons.
Structure
They consist of a pool of liquid and an air pocket above it. The depths of both of these are randomly generated, and are different for each column in the cavern. The liquid found in the pool can be any naturally generating liquid, either water, lava, or acid, and this is the same throughout the cavern. There are sometimes random blocks of stone or ores in the air pockets above the liquid. Obsidian can be rarely found if two or more caverns generate next to each other, as a combination of water and lava is needed for obsidian to form.
Nether Structure
In the Nether, caverns generate in a similar way, however instead of stone they have netherrack, and they can only generate with lava.
Gallery
An acid cavern
A water cavern in 1.4.34. This was before caverns were randomly generated, and always had this structure.
A water cavern.
