Free guided tours at Palazzo Vittoriano underground every saturday and sunday,
This site, remains of Emperor Trayanus' age, have been discovered in 1888, beneath the Ara Patriae, when building construction works begun.
The cellars were used as  nuclear bomb shelter during II World War.
You can book the tour at Venice Square entrance, half and hour before scheduled time:  11am in Italian and  12pm in English.
info: www.polomusealelazio.beniculturali.it