This is what happens to nickels and clad coins when they're exposed to dirt for any apparent length of time. I say dirt because they don't even need to be fully buried to start to look like this. I've found dimes on top of the soil of easily accessible planters in heavily traversed public areas (which is indicative they likely weren't there all that long) where one side was already starting to change color.
Soak it in water for a couple days, I bet half of that crud comes off. The 1939 is relatively common for an 80 year old coin. (120 million minted.) There is a rare reverse doubled die, (the letters in MONTICELLO). This isn't it.
Generally you would only use distilled water on coins. So you aren't getting chlorine and fluorine and other contaminants on your coin. But in this case, this coin is worth nickel. A dime in better condition that the one that you show, so it really isn't going to matter if you just soak it in tap water. As a rule you won't "wipe" it with a paper towel as that can impart scratches on the coin. Just pat or air dry. Some of this dirt may loosen and come off. Depending on if it is on the metal or in the metal. People have posted that it appears to be ED. Environmentally damaged, or a metal detector find. To me it just looks like age, heavy circulation and use, and regular dirt and not rust. Post a photo if some of the dirt washes off as a followup here.
Well there is a difference. Environmentally damaged/ rusty will be ingrained into the metal of the coin. While if it is just dirty from 80 years of circulation, that dirt can come off.
