Small service manholes for utilites are often death traps. Gases that push out breathable air and leave various non-breathable combinations in its place often seap in or collect in manholes from the earth or connected utilities.
Before the more restrictive rules on “confined space entry” utility workers of utility company people died quite often from just ducking down into a manhole to close a valve or take a measurement. I think Delphi had something similar.
When they collapse into the bottom of the hole, without a harness and a rescue tripod, there is often no way to get them out without ventalating the hole to a safe degree and then someone else entering the space to get them hoisted out.
In th case of this spot, a natural vent of such gas was noted and marked for a whole era in history — quite interesting.
Sanitary sewer systems make H2S, and CH4 in abundance, so any manhole is a potential death trap, but volcanic vents are not so easily recognized.
There was an old well on my grandfather’s farm. They would tell about when the whole community of women would gather there to wash their clothes. Over the years it showed signs of needing to be cleaned out. A man went down to clean it out and he came up dead. The well had filled with gas. I remember my grandfather constantly telling us kids to stay away from that well, which was by then surrounded by woods. I was fascinated by it and I’m sure I tried to get as close as I could without falling in.