There are currently 4 different layers of the underground, Identified by the background.
The layers are:
This is where the regular day and night cycle music plays, conventional mushrooms grow and the background wall resembles a regular craftable dirt wall. Zombies, Demon Eyes, bunnies, goldfish, giant worms, and all slimes excluding lava, black, and Mother slimes can spawn in this level. The dirt layer is the only layer in which you can place dirt directly on the wall with no supporting blocks.
Rocky Dirt LayerEdit
This is the layer where the music changes to underground music, which can be used as an identifier alongside the background itself to signify the true underground. This layer is where Glowing Mushrooms can be found/farmed, MOST Crystal Hearts are found, the majority of caves are found, and pools of water can be found. There are many different monsters that can spawn in this layer, such as the Skeleton, the Undead Miner, Mother Slimes, Black Slimes, Piranhas, Cave Bats and Giant Worms can all be found in this layer. The background wall here simply "exists farther back" and cannot be hammered away, but due to this, you do not need to hammer the wall away to make your own background walls.
In this layer, the background resemble the same as above, with the exception of the dirt look now being stone, and, like the previous layer, the wall "exists farther back" Same as last layer, but you will find much more in this layer, and close to the bottom of it, you can find lava.
This layer is a total conversion, with a background of stone with veins of lava coursing through it, as much Ash as the surface has Dirt, lots of Lava, some Hellstone, and new enemies: The Fire Imp, the Hellbat, the Lava Slime, the Demon and the Bone Serpent. Here, the music will change to an eerie tune. Also in The Underworld are small, randomly generated houses, made of Obsidian Bricks and Hellstone Bricks. The Hellstone bricks will still burn you, so have an Obsidian Skull equipped or have Obsidian Skin Potions. Inside these houses, you can find chests. This layer is not recommended for early players. The Wall of Flesh can only be spawned in this Layer.