What are those dark lines in the clouds?

If you look at this cloud closely, you will see a couple of dark lines. These are not distrails (dissipation trails). So what are they?

Look beneath the cloud, on the patch of blue sky. You notice a couple of contrails just left of the shadows. Even farther to the left you see a bright spot where the sun is. As the sun on the left shines on the contrails in the middle, the contrails form a shadow on the main cloud layer. This is an awesome picture showing how 3-D our atmosphere really is.