hiss sounds like a seam popping in an old boiler. May have been the boiler in the sign building next to the school. If the pressure valves fail there would be a huge explosion. We have a boiler out in our plant and we have to get it inspected quite often and there is special boiler insurance you have to buy to cover the special dangers. It would seem to me that if the boiler went then there would be massive burn injuries from the scalding steam and water.
I'm surprised there hasn't been
more boiler explosions in New York City considering many buildings in that city use steam heating systems in winter.
Hopefully this is an isolated incident because the results of a steam boiler explosion are definitely not pretty to look at, given what we've seen from steam boiler explosions on old steam locomotives.