I think there are plenty of mechanisms, not all well understood. Example: Thermal cycling (repeated warming and cooling) causes mechanical stresses. These grow microscopic cracks at imperfections / scratches / impurities in the plastic until there is a path for corroding chemicals (like water) to get in at the aluminium. Some CDs now use a gold layer instead of an aluminium one. I don't know how much longer these last. The above mechanism can be combated by keeping the discs in a temperature-controlled environment (a lake I would have thought would not be too bad for that but obviously it was...) - but mechanical stresses still occur when spinning up and down. Just my 2d worth. Philip.