I found this server called CaveMC and it is all about living underground. The entire overworld is a huge cave and there is no normal surface at all. Everything is generated with custom world gen so it still has different biomes and resources and structures but all of it is below ground.

The server is semi anarchy and survival so it feels pretty brutal. You have to be really smart to stay alive and keep your stuff. It is not peaceful at all and only really resourceful players will do well here.

Season 2 just started so it is a fresh start for everyone. There is also a new thing called the Wasteland Dimension. It is not just another biome. It is a big new area with its own map and special features. In the Wasteland you can find brand new gear that is powerful and exclusive. There are huge mega structures to explore and they hide secrets and good loot.

The Wasteland also has its own currency and that makes a new kind of player economy and trading. There is a new spawn made for this season so when you join you see a new starting area that fits the Wasteland theme.

Overall it looks like a tough semi anarchy cave world with a crazy underground overworld and a big new Wasteland Dimension for exploration and gear and trading.