The Biggest Underground City in the World is in Persia

The depth of this underground city varies from 4 to 18 meters and the different spaces formed by carving the rocks seem a little scary! To reach the underground city there were several different openings, some of these openings were located inside the houses of people and some others were located in important gathering places such as the main fort just outside the city. People could live in the underground passages and room for several days without the need of going outside.