This mod was created for the Interaction modpack. What it does is create 5 different dimensions that are exactly the same. These dimensions act as underground areas that exist below the Overworld. These dimensions have a large opening at y levels 150+ and have smaller caves that travel below y levels 128. If you are familiar with the Caverns mod, it generates a Huge Cavern on top of a Small Cavern.
See Images for examples.
This mod is an adaptation of the Cavern mod, therefore it contains libraries of the Cavern Mod that were created by the Cavern Mod Devs. I highly suggest downloading their mod and going to check them out at https://minecraft.curseforge.com/projects/cavern