Why does hugo refer to paris as the "central shaft of the mine"?

Victor Hugo refers to Paris as the central shaft of the mine because soon after the word of the July Revolution (1830) in France that put an end to the Bourbon monarchy spread throughout Europe similar uprisings followed in Italy, Germany, Switzerland, Spain, Portugal, Belgium, and Poland.

the answer would more than likely a.)falling crop prices  

hope the answer is right and ,either way have a nice day.