Cavern Layer

The  is a layer in The Underground. It is a lot like the rest of The Underground, but it has some changes. It starts from just below The Underground to The Underworld. The background changes when you enter the Cavern Layer. Gold Chests have better loot in them at this layer. This loot contains;
 * Archery Potion
 * Dynamite
 * Featherfall Potion
 * Flaming Arrows
 * GlowsticksCavern.jpg
 * Gold Bars
 * Gold Coin
 * Gravitation Potion
 * Hunter Potion
 * Invisibility Potion
 * Jester's Arrows
 * Night Owl Potion
 * Platinum Bars
 * Recall Potion
 * Silver Bars
 * Spelunker Potion
 * Suspicious Looking Eye
 * Thorns Potion
 * Throwing Knives
 * Torches
 * Tungsten Bars
 * Water Walking Potion
 * Band of Regeneration
 * Cloud in a Bottle
 * Enchanted Boomerang
 * Extractinator
 * Flares
 * Flare Gun
 * Hermes Boots
 * Lava Charm
 * Magic Mirror
 * Shoe Spikes

Lots of Gold Chests can be found on this layer inside of houses. Different enemies will spawn in too this list contains:

Hard Mode
 * Giant Shelly
 * Black Slime
 * Mother Slime
 * Baby SlimeSlime.jpg
 * Giant Worm
 * Skeleton
 * Cave Bat
 * Salamander
 * Crawdad
 * Undead Miner
 * Tim (Rare)
 * Lost Girl/Nymph (Extremely Rare)
 * Pinky (Rare)
 * Cochineal Beetle (Uncommon)Skele.jpg
 * Digger
 * Giant Bat
 * Armored Skeleton
 * Skeleton Archer
 * Rune Wizard
 * Ghost (During Halloween)

Notes:

 * Caverns are a great place to get loot since there are many Monsters and Gold Chests
 * The  takes up almost 75% of your world depending on world size.

Update Info:
PC Release
 * Added to the game