Rome is incredible. There are layers and layers and layers of history, waiting to be discovered, uncovered, restored and seen. Here is a recent opening (Just now in March of 2016) that you can't miss if you go to Rome: Basilica di Santa Maria Antiqua, in the Roman Forum area, at the bottom of the Palatine Hill.
"It was buried under rubble by an earthquake in AD 847 and was only rediscovered in 1900 during archaeological excavations. It has taken more than 30 years to restore its exquisite interior, which is decorated with multi-coloured frescoes of saints, martyrs, angels and emperors."
~ Telegraph (source link below)
Here is a link to a very nice Italian language site with more images of the Basilica.