Have you ever seen the science experiment (or magic trick?) where you get water supercooled to where it isn’t frozen, but then it freezes when you touch it, pour it, or otherwise disturb it? Apparently, ice crystals form around impurities or disturbances in the liquid and then “spread.” A similar effect can occur in metals where the molten metal cools in such a way that it stays melted even below the temperature that would usually cause it to melt.
[Martin Thuo] at Iowa University used this property to make solder joints at room temperature using Field’s metal (a combination of bismuth, indium, and tin). The key is a process that coats the molten metal with several nanolayers that protect it from solidifying until something disturbs the protective layer.