How to use Unity Terrain with 2D Physics?

Unity's 2D Physics are constrained to X and Y axes.

Unity's Terrain is oriented along X and Z axes, with the Y axis begin the "up" direction.

As Far As I Know, you can't rotate or change axes of neither of those. This means that you can't ever use these together in a game.

Is there any way to work this around? For example, it would be great if you could make the 2D Physics Engine work on other axes - X, Z in this case.