Cavern

The Cavern layer is under the Underground layer. It begins at about -850ft and can be noticed by the darker stone backdrop wall. Stone largely replaces dirt in this layer. Mother Slimes, Black Slimes, Giant Worms, Bats, and Skeletons spawn in this area. The Cavern layer is often rich in mineral deposits, the lower one travels the larger gem veins will become on average. Also it should be noted that lava begins to appear around -2100ft in small pools, which gradually become lava wells at around the -3000ft mark. You should however note that these measurements are different on different world sizes.

Monsters in the Cavern layer

 * Black Slime
 * Giant Worm
 * Skeleton
 * Mother Slime - spawns two or three Baby Slimes when defeated.
 * Dead Miner
 * Cave Bat
 * Tim

Unique Material Drops found in the Cavern layer

 * Hook (drops from Skeletons, Dead miners, and Piranhas)