Getting used to the smell doesn't mean it's safe though... And isn't the safe level of hydrogen sulphide "if you can smell it, its not safe"?
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...
No longer noticing the smell is NOT a good thing!