When you throw a bottle of water into molten steel, the result is the explosion. The moment the water bottle comes in contact with the liquid metal (which is 2500 degree Fahrenheit) in the ladle that holds it, the metal expands (and so does the water inside the bottle – 600 times), producing a shower of sparks.
The uploader of the video, Gregory Kroll noted that the explosion that took place in a plant at Kentucky, was not fatal and the surface of molten metal stood 10 to 20 feet below the camera.
Another user on Reddit reported that the thrower was probably at a safe distance and nobody was hurt. Also, the people around where the explosion took place were protected by their aluminum jackets.
Update: The uploader have the video removed, you can find the actual footage here.
Well, if you were around it without even wearing aluminum jacket, you would probably be safe, because the steel would instantly freeze and fall off, another worker in this plant notes. Here you can find the video taken from another angle.