In the following sentence from CNN:
Such fires were once plentiful in Azerbaijan, but because they led to a reduction of gas pressure underground, interfering with commercial gas extraction, most have been snuffed out.
The word "underground", I think it's an adjective, which modifies "gas pressure". so it's similar to "...they led to a reduction of the gas pressure which is underground".
If then, the sentence must be "...they led to a reduction of underground gas pressure" (underground precedes gas pressure)
Please help me to clarify the structure. I appreciate all your help :)