A labyrinth of little streets, alleys, and arches, this underground city was originally part of a fortress. It was built at the behest of Pope Paul III between 1540 and 1543 to confirm papal dominion over the city. When the papacy fell parts of it were destroyed, but much still remains. Begin your visit by taking the escalators from Piazza Italia and Via Masi. In the summer this is the coolest place in the city.
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