Can I create truly huge terrain in Unity?

I want to create a game with an absolutely massive map. Is it possible in Unity? I have dreams of large cities and open rural areas but I’m aware that my dreams might have to be scaled back what with hardware not being capable of it today. Any help is appreciated.

@Duck created a 100km-sq area using 100 terrains. See around 4min 50s on the last (Sim Focus) video on his recent blog post.