Underground

The Underground is the first layer below the surface in Terraria.^[1] It lies between the Surface and Cavern layers, starting at a depth of 0 feet.^[1] The layer has a distinctive brown background that cannot be removed with a Hammer.^[1] Natural blocks are mostly Dirt and Stone, with small pockets of Clay, Silt, and lower-tier gems and ores.^[1]

Treasures found here include Gem Trees, dye plants such as Green Mushrooms and Orange Bloodroot, and the Blinkroot herb.^[1] Enemies are more aggressive than on the surface, including Red and Yellow Slimes, Blue Jellyfish, and Giant Worms.^[1] In Hardmode, these enemies are largely replaced by Diggers, Green Jellyfish, and Toxic Sludge.^[1] Traps begin appearing in the Underground layer.^[1]

Mouse and Snail critters occasionally appear, and Fairies rarely guide players to nearby treasure.^[1] True underground biomes, except the Underground Desert, begin in the Cavern layer rather than the Underground.^[1]