I know what you might be thinking;"Caverns are underground they shouldn't be biomes!", But let me explain the surface area of the cavern is a plateau or mountain. Under the mountain there are giant caverns their minimum height being 50 max height being 100. Caverns 100 blocks tall are rare though. They have giant stalagmites and stalagtites. This creates large indents at the top and bottom. The bottom indents can be filled with water, lava and air. It would provide a easier way of finding ores.