Yes the data is stored on sliver top layer.
If that part gets messed up its all over.
The bottom of the disc is the clear layer, it can get scratch to hell and then you can buff it out to be clear and readable again.
I have to correct you on this. The data is actually printed in the clear polycarbonate layer. The top aluminum layer is there to reflect the laser (the artwork is printed on another layer on top of that), so if it's missing in spots, the laser goes through and the drive can't read the disc.
The reason the disc can get scratched and still be read is because the data is not located in the very surface of the polymer.