Underground

The Underground is the first layer below the surface. It starts at a depth of 0ft and can be recognized by its background, which contains many small rocks. Unlike Dirt Walls, this background does not allow the player to hammer holes for daylight or arbitrarily place blocks (i.e. if they don't join onto a nearby block). The music changes when the player goes a few feet deeper past the intersection of the rocky and rockless walls.

The blocks generated in the underground are about an even mix of Dirt Blocks and Stone Blocks. Underground deposits of sand can be found filling some of the caves and Clay blocks can be found too but are less common than in the surface.

The Underground is home to more aggressive enemies than the surface, as well as more treasure. In worlds generated after the 1.1 patch, this layer will contain traps, requiring that the player exercise additional caution. Worlds generated after the 1.1 patch will also contain Underground cabins in this layer.

In hardmode, the Underground may be converted by the Hallow, Corruption or Crimson, but it will not create Underground Hallow/Corruption/Crimson biomes, and monsters will mostly be a mix of surface Hallow/Corruption/Crimson and regular Underground enemies.

If a player continues downwards, eventually they will reach the Cavern layer.