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, often brown, 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, with Dirt and Stone making up the majority of the biome. Clay, Silt, and lower tier gems and ores can also be found in small pockets here. Some miscellaneous treasures can be found here, including Gem Trees (which drop Gemcorns, Stone, and occasionally their respective gems when chopped down); dye plants including Green Mushrooms, Orange Bloodroot, and Teal Mushrooms; and the Blinkroot herb.

The Underground is home to more aggressive enemies than the surface. More powerful Red and Yellow slimes will spawn down here (alongside the rare Pinky), while Blue Jellyfish spawn in the water and Giant Worms are found burrowing through the ground. In Hardmode, more powerful counterparts of these enemies will spawn, with the Digger, Green Jellyfish, and Toxic Sludge mostly replacing Giant Worms, Blue Jellyfish, and regular Slimes respectively.

Traps will also begin appearing in this layer. Underground Cabins, though not as common as in the Caverns, will frequently appear with very useful loot such as the Cloud in a Bottle, Hermes Boots, and Magic Mirror.

In general, not much time is spent here, but instead in the Cavern layer, which holds many more enemy spawns, treasures, and unique drops. Therefore, it's a wise idea to almost completely ignore this layer in favor of exploring the Caverns.

Occasionally, the Mouse and Snail critters will be found scurrying around. On the, Fairies have a rare chance of randomly showing up and guiding the player to a random treasure (such as Chests, Life Crystals, or Ore) nearby.

True underground versions of biomes, such as the Ice biome 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.

In terms of NPC happiness, the Underground is considered part of the Cavern layer. This means that the Clothier, Demolitionist, and Goblin Tinkerer NPCs all like living here, while the Golfer, Mechanic, Party Girl, and Pirate all dislike it.

Happiness
In terms of NPC happiness, the Underground layer is considered the same biome as the Cavern. This means that the same NPCs like the Underground as they do the Cavern, and when these NPCs are happy, they will sell the Cavern Pylon.


 * NPCs that like the Underground and Cavern:
 * Clothier
 * Demolitionist
 * Goblin Tinkerer
 * NPCs that dislike the Underground and Cavern:
 * Golfer
 * Mechanic
 * Party Girl
 * Pirate

History
Untergrund Földalatti_szint Souterrain 地下