Probably hydrogen sulfide gas. That’s the “sewer gas” you have probably gotten an occasional whiff of from a nearby sewer line or vent. Sort of a rotten egg odor. It will quickly kill you in any sort of confined space.
https://en.wikipedia.org/wiki/Hydrogen_sulfide
But many of the fighters and children who escaped the Warsaw Ghetto did so through the sewers, and they were a regular venue for running messages and supplies from outside the ghetto.