[DainBramage]

Hydrogen sulphide can kill you very quickly when concentrated.
I worked on a drilling rig in my youth. On one well hydrogen sulphide killed 3 roughnecks, each going over to save another and being engulfed it the fumes at the end of the blow out line.

[DainBramage]

Smells like rotten eggs. But soon you can’t smell anymore then fade to black.

[Myrddin]

H2S smells like rotten eggs below the toxic threshhold. Above that, it paralyzes the nerves in the nose. You can't smell anything. Also know as poor man's cyanide.

[delta7]

Could be from earthquake activity or ground movement. Could also be signaling a massive earthquake on the horizon.