Amazing Terrains in Unity?

Hi, i'm just wondering if it's possible to make realistic terrain in unity. Is it possible to make good terrain with the Unity Terrain Editor? or would you need a program that can make terrain?

WorldMachine 2? Terragen? Anything?

I'm also talking about hills, mountains, caves, underground areas amd the scale of the map.

Examples Darkfall http://www.youtube.com/watch?v=rtyRNWDhuXM

Red Dead Redemption http://www.youtube.com/watch?v=N5QWX371ruI

World of Warcraft

Thanks for reading!

You can't have caves and underground areas with the current system. You can either model your whole terrain as a mesh in a modelling application or develop a voxel based terrain system.

Regarding the realism, if you mean photo realism, you can always extract textures from satellite photos etc.