The Underground is the term for the vast area which stretches from around sea level ("Level" according to a Depth Meter) all the way down to the bottom of a world.
You can tell where you are in the Underground by the background, which corresponds to one of three layers: the dirt layer (brown, rocky background; not to be confused with Dirt Walls), the stone layer (grey background), and The Underworld (grey-black background interlaced with lava veins).
Ores, pots, chests, and water can be found anywhere underground. Demon Altars can also be found at any depth, though not very frequently outside of the Corruption. Lava replaces water near the bottom of the Stone Layer. The Underground is also the only place to find natural Glowing Mushroom biomes.
- Giant Worm
- Skeleton (stone layer)
- Most Slimes, excluding Green Slimes (rocky layer)
- Mother Slime (stone layer)
- Black Slime (stone layer)
- Undead Miner (stone layer)
- Tim (stone layer)
Chests may contain:
- Lesser Healing Potion
- Breathing Reed
- Enchanted Boomerang
- Hermes Boots
- Cloud in a Bottle
- Magic Mirror
- Band of Regeneration
- Angel Statue
- , or
- Gold, Silver, Iron and Copper Bars
- Suspicious Looking Eye(s)
- Throwing Knives
- Dragon Mask (Console Version)
Pots may contain:
- Lesser Healing Potions (Uncommon)
- Whoopie Cushion (Very Rare)
- Mushroom Grass Seeds (Very Rare)
Items not found in pots or chests:
- Life Crystals (found as background objects in the same manner as pots and chests)
- Statues (found as background objects)
- Glowing Mushrooms (found growing on mushroom grass, cut down with axes)
- Mushroom Grass Seedss (sometimes found by cutting down mushroom grass)
- The Underground is the only place where Glowing Mushroom biomes naturally occur.
In the dirt layer, one can place Wood Platforms and blocks freely on the back wall. This is not the case in the stone layer or below. You can only place blocks adjacent to other blocks, making pillaring the safest way out (unless you have a Grappling Hook, Ivy Whip or other flight/climbing item to use).
This may prove a problem for new players who've dug too deep. Instead of placing platforms (which require walls to the sides) it is advised that new players place dirt or other building materials below them to create a tower back up ("Pillaring").