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 (background with cave-like holes, little volcanoes and large amounts of lava pools / grey-black background interlaced with lava veins ).
Upon reaching the dirt layer, the music will change to the Underground theme. This music will remain until the Underworld is reached, at which point it will change to the "Eerie" theme, which is also played when a Blood Moon is present.
Ores, Pots, Chests, and Water can be found anywhere underground. Demon Altars/Crimson Altars can also be found at any depth, though not very frequently outside of The Corruption/The Crimson. Lava replaces water near the bottom of the Stone Layer. The Underground is also the only place to find natural Glowing Mushroom biomes.
- Main article: The Underworld
Basically, the Underworld is a hellish place full of demons and lava with its own soundtrack. It is located at the bottom of the world. In fact, you can reach the bottom of the world (an invisible unbreakable floor) by going to the bottom of the Underworld. You will need to visit the Underworld at least once in the game in order to unlock Hardmode as the Wall of Flesh can only be spawned in the Underworld. There are unique Shadow Chests in the Underworld that contain rare loot such as high-damage weapons and ammo. They can only be opened by Shadow Keys which are found in the Dungeon. Pots can also spawn in the Underworld.
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)
- Guide to Plant Fiber Cordage
- Herb Bags
Pots may contain:
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 Seeds (sometimes found by cutting down mushroom grass)
- The Underground is the only place where Glowing Mushroom biomes naturally occur.
- after defeating the goblin army you can find the goblin tinkerer somewhere underground
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). Rope is another extremely easy way to get out. If you throw a Rope Coil upwards, it should attach to the roof, giving you an easy way out.
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").