The Mushroom Kingdom has an green grime scattered on the ground with mysterious giant inflated mushrooms scattered across the lands. Later on, the green grime was confirmed to be nuclear waste left by Philips. Many years later, Mario and Luigi revisited the poisoned wasteland and reverted it back to it's original state and turned the mushrooms into Toads. Fat Mario, Bowser and Gay Luigi were nowhere to be seen by the time then. A old Bowser's Sourpuss Bread package was rumored to be recovered, but this has yet to be confirmed.
The radiation causes whoever enters it to ocassionally gain 200 pounds, especially when being confronted with specific issues. As another side effect, it may cause brain damage proven by the occasional moment of silence from Fat Mario. The radiation can even leak out of your CD-i and cause you to mutate into a terribly animated character and spout gibberish about spaghetti and toasters.