Some interesting facts about the Moscow's M.
The Metro could have appeared as early as 1875 in Moscow, but the Russian Orthodox Church blocked it by claiming that "a man, created in the image of God, cannot humiliate himself by descending into the underworld."
In the October of 1941, during 2nd world war, the soviet government was preparing complete liquidation of the Metro. It was supposed to be flooded but the government order was soon abolished.
The total combined length of the M is 300km or 186 miles.