Experiments were conducted in which a molten copper droplet was released into a pool of water. Spontaneous vapor explosion did not occur when the water temperature was 50°C or higher. Spontaneous vapor explosions, however, occurred at a rate of 70% when water temperature was 20°C A series of high-speed video images explores the triggering process of spontaneous vapor explosions: (1) when a molten copper droplet is released into pool of water, a vapor film forms and separates the copper droplet and the surrounding water that is a boiling film, (2) a filament of molten copper grows from the surface and deforms the vapor film, (3) since the filament of molten copper has a small heat capacity and is rapidly quenched, the vapor film condenses along its surface, and the molten copper droplet around the filament directly contacts water, and finally (4) triggering of spontaneous vapor explosions occurs from the filament to the whole molten copper droplet. When filament growth was observed, it triggered a spontaneous vapor explosion in almost all cases. When filament growth was not observed, spontaneous vapor explosions were not observed and vapor films therefore stably formed around the molten copper droplets. The authors thus conclude that filaments from the molten copper trigger spontaneous vapor explosions in highly subcooled water.