To begin your trip in underground Perugia, head to the Museo del Capitolo di San Lorenzo, next to the cathedral of the same name. Check visiting arrangements before you go as booking is required. (http://www.cattedrale.perugia.it/?p=26 , click here to find information regarding opening times and prices).
The visit begins from the cloister of the Canonica, via a side staircase. You will find yourself in the Sala del Conclave, where four Popes were elected: Honorius III, Honorius IV, Celestine V and Clement V. The large room appears to be divided by a wall, built in the middle of the last century to support the floor of the cathedral above.
Leaving the room, follow the route indicated, which will guide you to the remains of the Etruscan walls, built to make the acropolis of Perugia appear even more majestic. Here there was a temple, perhaps dedicated to Juno-Hera (in Etruscan, Uni). You will discover how in this part of the town has been in use, without interruption, since Etruscan times. Proof of this can be seen in the vault, of medieval origins, built in the foundations of the Cathedral which can be visited.
Still following the route, walk on the ancient cobbled road, first used by the Etruscans and later the Romans, on which the ruts left by cart wheels are still visible today. Continue your trip following the ancient Roman road and you will find yourself below Piazza Cavallotti.
If you want to make your trip to underground Perugia even more exciting, check the CAI website which organises exciting and fascinating "walks". You can be a speleologist/archaeologist for a day and see the Etruscan well, the underground areas of the Cathedral, the Postierla della Conca, a secondary entry gate to the town, near the Roman aqueduct, that allows access to the deep underground passages intended for pedestrians or also, according to some, for flood waters, as well as the air-raid shelter- a historic place that joins Corso Cavour to Rocca Paolina.(http://www.caiperugia.it/)