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KANSAS CITY, Mo. -- Witnesses could hear the explosion, and the sound of sirens for miles, as crews battled a magnesium fire at a scrap metal yard early Saturday morning.

It was near 12th and Spruce in KCMO.

Magnesium burns really hot, and becomes explosive when you add water. Firefighters didn't know they were dealing with it at the time, so they used water.

The fire involved four semi-trailers. Crews eventually had to let the fire burn itself out.

No one was hurt. The cause of the fire is unknown.