This server is called CaveMC and it is on Season 2 right now. The whole overworld is actually underground so there is no normal surface at all. Everywhere you go is caves and underground biomes and it all uses custom world generation made by real Minecraft developers.
It is a semi anarchy survival server so the world is pretty brutal. People can fight and raid and only the most resourceful players really last. It still feels like vanilla but it is enhanced and way more intense because you are always in caves.
Season 2 also adds a new place called the Wasteland Dimension. It is not just one biome. It is a huge new area with its own custom map. In there you can find brand new powerful gear that changes how combat and survival work. There are mega structures that are massive and they hide secrets and good loot.
The Wasteland has its own special currency and that means a new kind of economy and trading between players. There is also a new spawn that fits the Wasteland theme and sets up the challenge when you first join.
Overall this server is about surviving underground in a harsh semi anarchy world and then pushing into the Wasteland Dimension for better gear and riches if you are brave enough.
