I work in an environment that deals with Hydrogen Sulfide
on a regular basis.
When it's pure there is no odor, or smell, whatsoever...and it will kill you dead.
That's if you manage to breathe the pure concentration of it.
I've actually read stories of men dying very soon after they smelled it.
Which means the concentration wasn't pure, but somewhat dispersed in air.
They must of had a large exposure.
Hydrogen Sulfide, or H2S, is an extremely poisonous gas.
There is no joking around, this gas will cause certain death.
When it is introduced into the atmosphere, it reacts causing the "rotten egg" odor.
But even then a small dose will cause brain damage or even death.
I believe it is best to steer clear of the gas, anytime it's presence is known.
That being said...
You may be smelling a sulfuric gas release in the air.
Caused by anything from sewage waste to fissure in the Earths crust.
If you can smell rotten eggs, then the concentration may be low enough...
...As not to cause certain death or brain damage.
edit on 10-1-2013 by havok because: Spelling!