Its distinctive background can recognize the Underground, 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 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 handy 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 enemies, spawns treasures, and unique drops. Therefore, it's a wise idea almost entirely to ignore this layer in favour of exploring the Caverns.
Occasionally, the Mouse and Snail critters will be found scurrying around. On the 电脑版和 移动版, 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 begin in this layer, but in the Cavern layer (except for 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.
- 电脑版 22.214.171.124：
- Added the alternative Otherworldly Underground track, originally from Terraria: Otherworld, which plays once the player has entered the 05162020 seed and pressed the "Music" option in the Party Girl's menu.
- Blue Jellyfish now have a 1 in 50 (2%) chance of dropping the Blue Jellyfish Kite on Windy Days.
- Cavern Pylons will now be sold by any happy NPC in the Undergroun for 7 50 to 8 50. While any number of pylons can be placed Underground, only one Cavern Pylon can be functionally used per world.
- Enchanted Sword in Shrine background objects now have 1 in 10 chance of dropping the Terragrim instead of the Arkhalis.
- Fairies now have a rare chance to appear randomly and guide the player to treasures such as Chests, Life Crystals, and ore.
- Gem Trees now have a rare chance of generating and, when chopped down, drop Gemcorns, Stone, and rarely Gems.
- Gold Chests can now once again spawn outside of Underground Cabins.
- Kelp will now grow on Sand underground.
- The Enchanted Boomerang no longer generates in Gold Chests, as it is now crafted instead.
- The Mace can now be found in Gold Chests.
- 电脑版 126.96.36.199：
- Enchanted Sword in Shrine background objects now generate rarer (as they mostly generate in Enchanted Sword Shrines now) and have a 1 in 10 chance of dropping the Arkhalis instead of the Enchanted Sword.
- Gold Chests no longer generate outside of Underground Cabins.
- Gold Mice, the gold equivalent of normal Mice, now have a very rare chance to spawn.
- Pinky will now drop Pink Gel upon death instead of normal Gel.
- 电脑版 1.2.4：
- Exclusive fish can now be caught Underground, including the Armored Cavefish (used to craft the Endurance Potion), Golden Carp, Specular Fish (used to craft the Recall Potion), and Stinkfish (used to craft the Stink Potion).
- Multiple Angler quest fish can now be caught Underground (only when the Angler asks for them), including the Batfish, Bonefish, Dirtfish, Jewelfish, and Spiderfish.
- 电脑版 1.2.2：新的《地下备选曲》配乐。
- 电脑版 1.2：
- Added multiple new drops:
- Added new background.
- Enchanted Sword in Shrine background objects will now very rarely generate and have a 1 in 3 chance of being a real Shrine, meaning it will drop the Enchanted Sword item when broken.
- Extractinator, Flare Gun, and Shoe Spikes can now generate as main loot in Gold Chests.
- Green Mushrooms, Orange Bloodroot, and Teal Mushrooms now occasionally generate in the terrain.
- Ores now have a chance to be replaced by an alternative ore, meaning Tin, Lead, Tungsten, Platinum, and (in Hardmode) Palladium have a 50% chance of generating in the Underground in replacement of Copper, Iron, Silver, Gold, and Cobalt, respectively.
- 电脑版 1.1：
- Added the Hardmode Underground with four new enemies:
- Diggers, the more powerful equivalent of Giant Worms which will mostly replace them. Like Giant Worms, they also drop the Whoopie Cushion.
- Green Jellyfish replace Blue Jellyfish.
- Toxic Sludges mostly replace Blue, Red, and Yellow Slimes.
- Alongside these enemies, the Possessed Armor, a Hardmode equivalent to Zombies, will also spawn down here.
- Cobalt Ore will now generate here after the player has smashed a Demon Altar (other Hardmode ores generate deeper in the Caverns).
- Traps such as Boulders, Dart Traps, and Explosives now have a chance to generate here and surprise the player.
- Underground Cabins, which hold Gold Chests (alongside some Cobwebs and furniture), now generate here.
- Added the Hardmode Underground with four new enemies: