(click to enlarge the pictures) Some background and more details: - Coin was stored in the 2x2 for about 10-15 years. - For about 3 of those years, the coins were in a non very friendly environment. High humidity, lots of dust. - This is mainly happening to the BU coins. - After taking the coin out, some of the substance stuck to the coin, but most of it remains in the plastic of the 2x2. (Picture 3) - A soft wipe removed the substance from the coin and it looks BU again. (Picture 4) - Pressing the plastic window against the coin, without opening the 2x2, makes the "fog" disappear.