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A small amount of mercury amalgamates itself into an aluminum I-beam and destroys it from within. Gallium scratched into the surface allows the mercury to penetrate the protective oxide layer that normally surrounds anything made of aluminum. This is a time-lapse video, the action takes about two hours in real time. The powdery oxide is falling off, what you don't see is a significant pile of it building up underneath. This is the original Pop Sci article on the topic:
