I missed this in the link:

Let’s start with the basics. Peanuts are not actually a nut at all. They are classified as a legume, just like a bean or a chickpea. Unlike almonds, which grow on trees, peanuts grow underground in the soil. During their growth, while the warm, moist earth surrounds them, they are often exposed to mold and fungus. Unfortunately, the fungus causes contamination of the peanuts and ends up producing cancer-causing compounds known as aflatoxins.