As I heard, Russians had an experiment several years ago, they buried a case od CD discs into the ground with some information on them, without using them, and then took them out 2 years later and the disc (CD-R)was unreadable. CD-RW seem to last for full 5 years, in theory DVD's should last for about 50, the testing is not over yet, though. In that case, 2 questions:

1. Why does the disc become unreadable with time, if there is no physical force acting on it (bending, scratching, etc.)
2. Why do my old CD-Rs work (althouth no as good) on my computer that are 3-4 yrs old? Does it have something to do with the location which they've used for "burial" of CDs?

Difficult to say, for sure. If they buried them near Chernobyl, that could be a factor...

Do you have a link to the source of the study? Then we might all read the particulars and be in a better position to comment.

Unfortunately - not. I've read it some 3-4 years ago, and didn't think of it that much at that time. My guess is that the burial place was high in magnetic metals and thus the information loss.

I dont think magnetism affects CD's as theyre optical disks and burned with lasers and such not using a magnetic method like floppy disks.

My guess is one of two things, either the device they used to read them with was incompatible, very unlikely. Or the disks were damaged by things like moisture and/or bacteria etc that could have changed the etchings which were recorded on the disk in a similar way to scratching them.