Yanvar station in Baku, Azerbaijan (Public domain)
From Petersburg to Novosibirsk, the former Soviet Union boasts some of the world's most unique and ambitious metro systems. Built at command-economy speed, these metros were the result of a political ideology that placed heavy emphasis not only on infrastructural prowess and efficiency (could communism be, dare we say, green?) but also on military readiness. Indeed, subways were seen as a crucial transport link for soldiers, and some of the USSR's metro stations were intended to double as bunkers in the event of war. Even if many of the subways have suffered from the financial and political turmoil left in the wake of the Soviet Union, they are still crucial modes of public transit -- and awesome eye candy for underground architecture buffs.