I'm going to guess that the place would trigger confined space air supply rules going in there for any work. I don't think they would ventilate a natural wonder like that as it would change the moisture and temperature conditions and possibly screw everything up given enough time.

Generally, people do not enter toxic environments without an excellent reason... And then we have geologists.

Burritofarts.

Probably just carbon dioxide but still dangerous. (pioneerwalks.com.au)

The nearby residents ate too many chimichangas.