What Causes Crepuscular Rays?

I was watching the sun today-poking in an out of clouds. All of a sudden, there was a brilliant series of rays coming through the cloudes-what are the sun rays reflecting from? Atmospheric dust?
They really are spectatcular-too bad I din’t have my camera.

They’re columns of sunlight; other adjacent rays of light are either blocked or dimmed by clouds, hills, trees or what have you. The light is then refleced off dust or water droplets. Here’s an example from wikipedia where the rays are caused by windows.

The question is answered, but here is the obligatory link to Atmospheric Optics (fantastic pictures!).