Originally Posted by
Boltorano
This sounds like disc rot, a problem that occurs when the disc is made cheaply or with the wrong materials, or exposed to direct sunlight for long periods of time. On a regular disc it will show up as a dark spot on the bottom side of the disc or spots where you can see light coming through the disc label. I hear that on PS2 discs with the dark coloring it shows up as lighter spot than the normal color of the disc.
This is an oxidation of the data layer of the disc, and any data on that part of the disc is lost forever if that's the case with yours.