@GothicSoul posted the link below in a thread on the rookie forum recently. This study contains some very unique and very thorough data on workers (and would-be rescuers which about a third to a half of all of these victims in these studies were would-be rescuers, btw) that have died in confined spaces. NIOSH has pulled data from a large cross-section of industries, both rural and urban. It's further broken down into a large amount of subsets (what kind of confined space, specific cause of death, like oxygen deprivation [asphyxiation] vs poisoning from various other toxic gases, "drowning," - like in grain, for instance). Anyways, I highly recommend checking this out if you partake the underground adventure varieties, or even if confined spaces are in your job description:

https://www.cdc.go...03/pdfs/94-103.pdf

I'm not by any means a PhD, but I had to scour countless studies in college and for what it's worth, I'd say the validity/reliability factors in the way this study was conducted is very good.

As far as gases in confined spaces go, I don't fuck with that shit. Unless I'm in there with someone actually trained in using SCBA gear or any other similar equipment, and in responding to these scenarios, I try to veer away from that stuff. At best you'll do an okay job and everything is usable. At worst, you get overconfident that you have a gas mask/air tank and it becomes a detriment and you die from the gases.

As far as draining in general goes, I'd personally try to find people that have explored said drain before, or research the drain and see how closed off it it/if any big well-known hazards exist, or something of that nature before going down there.

EDIT: added some stuff