CaveMC is a semi anarchy survival server with an enhanced vanilla feel. The big twist is the entire overworld is underground. There is no surface at all so you live in caves forever.
The world uses custom generation made by real Minecraft developers. You can still find biomes resources and structures but everything is under the ground. Survival is meant to be brutal so you have to be smart to last.
Season 2 is out with a fresh start and new additions. A big new feature is the Wasteland Dimension opening on November 8th. It has new gear to fight for huge mega structures with secrets and its own currency for a player driven economy and trading. There is also a new spawn and a custom generated map with special features.
