The Troll Caves beneath the Highlands take several forms. First, there are normal tunnel-style caves throughout the area. These resemble larger versions of normal minecraft caves, speckled with Trollsteinn and draped with Trollvidr.
Following the tunnels, you may find several types of large cavern. The large central cavern seems to have special significance to the trolls. The area is dominated by a massive vault made of Giant Obsidian. Nearby caverns branch off of the central cavern, and are filled with vines, fungus, and several types of stone formations. You may occasionally find smaller vaults, made of normal obsidian, containing treasures.
At the end of a series of branching caverns, you may find the garden caverns, with ground made out of mycelium, rough dirt, and Uberous Soil. Inside a vault in the garden caverns you will find several treasures, but always some Magic Beans.
The troll caves contain mostly trolls, but also witches, skeletons and creepers.
Adventurers returning to the troll caves from the Cloud Cottage above, may be able to open the central vault with the Giant's Pickaxe. Inside the central vault is the Lamp of Cinders, needed to burn through the Thornlands on the way to the Final Plateau.