Underground

The Underground consists of four layers: The dirt layer, the rocky dirt layer, the stone layer and The Underworld layer. While The Underworld is considered its own environment, it is still technically an underground layer. The underworld has it's own boss known as the Wall of Flesh. The Wall of Flesh can be summoned by throwing a guide voodoo doll into the lava.

Dirt Layer
This area, despite being underground, is still considered to be above sea level. Therefore, the music will remain the same as the biome that's above it, and any monsters found within the dirt layer are also found on the surface, including Green, Blue, and Purple Slimes, Zombies and Demon Eyes (if it's nighttime), or, if there is Corruption within the area, Eaters of Souls and Devourers.

Rocky Dirt Layer
This layer is located just below sea level. You can tell when you've reached this layer when the background changes to a mix between dirt and stone, and when the music changes to the underground theme.

Stone Layer
The stone layer comprises most of the Underground. After moving below a certain depth underground, lava starts to appear in place of water.