Come on guys, magnesium doesn't spontaneously combust in water. The part people are referring to about the water is you can't use water to put a magnesium fire out. That won't work. You also can't use an "A", "B", or "C" fire extinguisher. All the fire extinguishers you see are "A", "B", "C" or a combination of those. All you can use on a magnesium fire is a "D" or dry sand. You probably don't know anybody that has a "D" fire extinguisher and dry sand seems to be hard to find when you really need it. Magnesium castings don't burn easily. Magnesium shavings will burn instantly. Magnesium dust is explosive.