They have had groups camping near Mammoth Lakes die from the high CO2 concentration from volcanic vents.
Very active area; the ground there is rising constantly.
Very few people realize that the entire valley containing Mammoth Lakes and extending all the way across 395 to Crowley Lake is one giant caldera, and Mammoth Mountain is just one lava and cinder dome. When you drive down the Owens Valley, you pass through vast lava fields and a number of cinder domes,
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.