It’s a compound found in a lot of oil and gas drilling. There are all kinds of nasty toxic substances found in the media that comes up from oil and gas wells. H2S is one of the worst. Companies use a lot of oversight and caution on production equipment for “Sour” wells.
It is also found in some groundwater wells and apart from the nasty odor, it’s rare for an accident of this nature to occur from a drinking water well.
Thank you both for the info, I was not aware of this gas. Very scary!