First off, this is not the "hot slag" you're looking for online. It's also not as much fun to toss off a bridge, but it is pretty impressive.

According to my railroad buddies, when they replace a section of worn track, the usually join the sections together by melting them into a solid piece with a chemical reaction called "thermite welding." It's super cool, google the process and check it out.

So after welding the two rail sections together, you're left with the impurities called slag. It is both used to shield the welded section and a byproduct of the process. Naturally, it's hot. Red hot. So what do you do with it? It's literal garbage that normally gets tossed in the bin... but if you happen to be working on an old timber-rail bridge and have the space to slip a heap of it down to the depths below, why wouldn't you?

When the slag hits the water surface, it explodes. I suspect it has something to do with the slag fracturing due to the instant temp change (same as pouring cold water on a hot engine block) or perhaps the slag is so hot, there is an instant formation of steam... or both, I don't know and I'm not interested enough to google it. It is fun to watch though.