The Biggest Underground City in the World is in Persia

The most striking of features is the sophisticated air conditioning system, which keeps the three floors cool and ventilated. Other impressive aspects include the manner in which the corridors have been carved at an angle for defensive purposes.  If an enemy was to appear at any of the entrances, the angle would prevent them from being able to see down the corridor, thus allowing the inhabitants’ time to escape or to ambush the enemy.

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