It seems well accepted by people who work in underground environments, often confined spaces, that there is the possibility of finding gases there, such as methane and hydrogen sulphide, but does anyone actually know how they got there?
Cue the music and call for Sherlock!
Lets start by looking at the stuff which makes up, well, everything!
People, and leaves and dog poo and cheese, could be some of Julie Andrews favourite things, but in this case they are just examples of the many things which are made up of stuff. In chemical terms stuff is just a bag full of elements such as carbon, nitrogen, sulphur, and hydrogen and everything is made from it.
When things die they begin to decompose, the picture shows the stages of an apple doing exactly this, and as the apple decomposes the stuff of which it is made changes itself. If the apple is in the fresh air (or an oxygen rich environment) then the stuff will decompose by pairing up with the oxygen. This is what they like to do best.
Carbon will grab the oxygen and become carbon dioxide, sulphur will grab the oxygen and become sulphur dioxide and nitrogen will grab the oxygen and become nitrogen dioxide. Now these aren’t innocent gases, they are toxic in their own right, but if this occurs in the open air then they will simply join the gases in the atmosphere and effectively disappear.
If, however, this is happening in a sealed underground chamber, then the amount of oxygen present is limited, so as the oxygen gets less the carbon can grab less of it and becomes carbon monoxide – another highly toxic gas. When the oxygen runs out all together, then the only partner left for the stuff is hydrogen. The carbon grabs hydrogen and becomes methane (highly explosive), the sulphur grabs the hydrogen and becomes hydrogen sulphide (very toxic) and the nitrogen grabs the hydrogen and becomes ammonia (also very toxic). Every element needs to have a partner!
The longer the underground chamber has been sealed, the more likely it is that the oxygen has been used up (which can also cause a low oxygen asphyxiating atmosphere), and the more likely it is that the elements present have combined to form these toxic and explosive gases. So, these gases do just appear out of thin air, and as with so many things my dear Watson, it is elementary!