Invasion of the data snatchers

Several of my CDs have developed a murky sort of coating, which looks a bit like condensed steam. The disks in question have now been rendered unreadable. What is it exactly?

I hate to be the bearer of bad news, but what you've got there is a reflective aluminium disk layer munching CD fungus. Usually the malicious critter attacks the edge of the disk where the aluminium is exposed and then eats its way inwards, though it is not uncommon for it to also attack the polycarbonate sandwich too. The aluminium layer is an essential component of the disk because its reflective surface provides the means of bouncing your CD drive's lasers back into the unit to allow it to read the data. Once the aluminium layer is gone, there is nothing to provide this feedback and the disk is rendered unreadable.

So, what can I do about it? Well not a lot actually. The disks which have already been damaged are probably beyond repair, but you can prevent the same thing from happening in the future by only buying writable disks which have a gold reflective layer. The data stored on these disks is protected from the fungus because the gold layer is immune to its ability to munch through the coating.

The only other option you have is to try to seal the exposed edge of your CDs using a clear varnish to prevent the fungus from getting a foothold, but this can also be problematic for obvious reasons. Diamonds may be forever, but CDs aren't.