This is a paid asset, but now you can download Hand Painted Cave Pack Free.
Detail this asset from Unity Store: Original Link
Hand Painted Cave Pack v1.1
Update 1.1:
Removed all duplicated materials and animator components.
All meshes are lightmap enabled.
Added prefabs for easier level building.
Removed standard assets.
Compatible with unity 5!
Stylized assets to create your own fantasy cave or maze environment!
Videos:
Video walkthrough
Video walkthrough 2
Web demo:
Web demo
Features:
Includes an example scene and prefabs to get you started
Models range from 40 to 674 triangles.
Cave parts based on unity units (compatible with pro-grids)
Crystals, mushrooms with customizable colors.
Rocks, ferns, vases, ladders, carts, structures.
Particle effects, rainfall, torches, animated flags.