"Bruce L. Bergman" wrote

Bad air from oxygen depletion or hydrogen sulphide and explosive
vapors are a much more imminent hazard, and all you need to do to get
killed by them is climb inside without testing and ventilating the
space first.

Yep, there is a reason you need a permit to climb down into such places.
There are dangers that can kill, if you do not have the proper training.
--
Jim in NC