boat sju
Contributor
Hydrogen sulfide is poisonous - but usually harmless in the quantities we experience smelling marsh gas. I believe we go "nose blind" to it at higher concentrations, making it dangerous.
I was taught that you go "nose blind" pretty much just before it kills you.
I've experienced the same thing diving in (parts of) quarries too. I thought it was interesting how you can smell under water. I guess it's not really a big deal but who wants to dive in swamp water.