Beyond boundary flat extension terrain?

Hi!

This is my first project in unity, so I'm not fully aware of what is possible with it. From the impression I got by looking at tutorials and seeing some game projects, I see that a unity game is constructed out of scenes that have a limited environment; usually a square area within which you have a terrain and some game objects scattered across it. If you try to walk past this square area, an invisible wall will stop you or you'll just fall into the abyss.

The game I'm making involves a robot character with full flight capabilities to navigate in space and on planet surfaces, like gundam or armored core style mechs.

These pictures explain what I'm going for:

As you can see, in the middle there is traditional play map with possible scenery, objects and custom terrain. Past the bounds, there is nothing. However, I want the player to be able to travel past these bounds as the arrows indicate. I need this because the game has high-speed combat and I want to avoid players from colliding with an invisible wall if they reach a boundary.

For space is seems pretty simple to construct, just put all the game stuff within the cube of play-area, but allow the player to leave its bounds. But for planetary levels I need to have "beyond-boundary flat extension terrain", so that players would not fall into the abyss when crossing the bounds of the custom terrain of the play area. The custom terrain on planetary levels would be tailored by me to make sure it merges with the flat extension terrain.

This game is the perfect example how it would look visually:

http://www.youtube.com/watch?v=eke2X8M5GQI&feature

So yeah, the question is:

-How can I achieve this result using Unity?

-How can I create beyond boundary infinite flat extension terrain around the planetary level's custom terrain?