Underground

The Underground is the first layer below the surface. It fills the area between the Surface and Cavern layers, beginning at a depth of 0ft and extending downward.

The Underground can be recognized by its distinctive background, which no longer contains naturally-placed Dirt Walls like surface terrain, but instead displays a unique backdrop with no naturally-placed walls. Players cannot remove this backdrop using a Hammer, nor can they place blocks in midair, as no wall generally exists here (though players can place their own walls, just as they can above surface terrain). Natural Underground blocks are generally similar to those in the surface.

The Underground is home to more aggressive enemies than the surface, as well as traps, Chests, and Underground Cabins.

True underground versions of biomes, such as Underground Snow or Underground Corruption, do not actually begin in this layer, but in the Cavern layer (with the exception of the Underground Desert, which begins in Surface terrain and extends through the Underground and a portion of the Cavern). In Hardmode, some blocks in the Underground can be converted into Hallow, Corruption, or Crimson.

History
Untergrund Souterrain 地下