The least known of the Museo Nazionale Romano's four museums, the Crypta Balbi is built around the ruins of medieval and Renaissance structures, themselves set atop the ancient Teatro di Balbus (13 BC). Duck down into the underground excavations, then examine artefacts taken from the Crypta, as well as items found in the forums and on the Oppio and Celian Hills. Note that there is sometimes a €3 exhibition supplement.