Computers can accumulate a large amount of dust and gunk in harsh environments. The pictures below are of an Apple Power Macintosh 8500. This computer was bought new in 1996. It was my primary computer in high school. When I moved out of my parent’s house I set it up for them to use. It spent a few years in a very dirty building; a building where dirty ethylene glycol is recycled into full strength automotive antifreeze. I suspect that most of the gunk we found in the case is from being in that environment.
Believe it or not this computer was actually running fine in this condition!