Big Caves

A cave (also known as cavern) is a common terrain feature that generates in the Overworld and the Nether. Caves are usually found underground. They are open spaces of various sizes and shapes that often intersect with each another or with different generated structures, creating vast cave systems. They usually feature plenty of ores and are great places to gather resources, but can be dangerous because of the hostile mobs that spawn in the darkness.

Description
These are large subterranean systems, found deep within the overworld, often close to bedrock. They hold large deposits of essentially every known type of ore - with rarer materials, such as diamonds and emeralds. Note that emerald ore is exclusively generated in mountains biomes. Such systems contain a number of lakes of all types, as well as the presence of major lava and water flows. There is little to no difference between small, medium, and large caves other than their size and complexity. Mega caves are huge spacious caves that are extremely rare to be found. They expose many ores and can potentially expose mineshafts. Unlike large caves, mega caves can range from 20 blocks and above in length. They can also be possibly bigger than a small ravine. Mega caves that generate low enough can form giant lava oceans.