Getting used to the smell could be an extremely bad sign, in fact.

With hydrogen sulfide, olfactory fatigue can occur at 100 ppm, and paralysis of the olfactory nerve has been reported at 150 ppm[1]. Those levels are considered "immediately dangerous to life and health (level that interferes with the ability to escape)"[3]

So you might think the gas is gone or that you're "used to it", but you're being poisoned and are at risk of losing consciousness and dying.

However, according to [2], "the atmosphere in [Sulfur Cave] can reach concentrations of up to 14 ppm of H2S". According to [4] that's below the "acceptable ceiling concentration" of 20 ppm, with up to 50 ppm for a 10-minute period being acceptable for an 8-hour shift. So these levels are in the "not great, not terrible" category. Wearing a mask is highly advisable.

[1] https://www.ncbi.nlm.nih.gov/books/NBK208170/

[2] https://subtbiol.pensoft.net/article/162344/

[3] https://www.osha.gov/hydrogen-sulfide/hazards

[4] https://www.osha.gov/laws-regs/regulations/standardnumber/19...

Yes, that was my thought as well. As a youthful experimenter, I once did not clean up properly after myself and stopped smelling the H2S which was permeating the room (Back then, the recipe for H2S was in chapter 1 of the Chemistry sets. Good times!)

No longer noticing the smell is NOT a good thing!