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 on the surface layer, players can't hammer holes in the background for daylight on this layer and below, nor can they place blocks away from other blocks, because the "wall" (with a few exceptions) is purely aesthetic. Spiders cannot climb on this wall. Players can, however, place their own walls just as on the surface layer, but again, this must be done adjacent to existing tiles. The music changes when the player goes a few feet deeper (around 58 feet below) 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, blocks in the Underground may be converted by the Hallow, Corruption or Crimson, but true Underground Hallow/Corruption/Crimson biomes begin at the Cavern layer and below, and so will not occur this close to the surface. In this case, 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.