Strange deposit or growth inside “sunflower” iMac cover?

[jido]

When I removed the bottom plate of the iMac, I found this strange deposit. It is about 0.5cm tall and circular. What could it be?

 

[jido]

After removing it you can see the Apple logo and a circle:

 

[AphoticD]

From what I recall, the bottom plate was used for heat dispersion... it could be an old thermal pad?

 

[OrenLindsey]

Probably thermal paste if I had to guess

 

[jido]

Thanks, that does make some sense.

 

[evanboonie]

Normally, this is just a rubbery plastic disc that I believe serves to support the bottom or help hold the AirPort card in place. For some reason that I have never been able to figure out, they get... weird, sometimes. Mine always has a honey-like substance on it when I take it apart after not opening it up for a while. The sticky fluid also builds up on the bottom of my AirPort card. I have never been able to figure out what this is, but it always comes back, even after cleaning it completely. There is no evidence of a fluid anywhere else in the machine. My best guess is that the polymer used is very hygroscopic and breaks down in humid climates over time.