These are remnants of the three pagan Roman temples that the basilica was built over-top and surrounded by. To give you a sense of what is meant by this, here is an archeological depiction showing the three former temples overlaid over a photo of the present Christian basilica.
These temples come from Rome's Republican era, dated to the third to second century B.C. dedicated to Spes, Janus and Juno.
As for the Christian basilica, its origins are dated to the sixth century though the present structure is the result of a substantial rebuilds which took place at the end of the sixteenth century as well as many restorations during the 19th century -- including the ciborium magnum itself which dates from that period.
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| A view behind the altar into the apse |
Amongst the spoila you can find within the basilica, ancient, mismatched columns line the nave and beneath the high altar you can find ancient bath made of green stone that has been repurposed and now contain the relics of the Roman martyrs Mark and Marcellinus, Simplician, Faustinus and Viatrix.
A beautiful coffered ceiling was added to the basilica in 1868, one of the more impressive ones in the city of Rome to my mind, with its beautiful combination of blues, golds and accented by reds in the heraldry. They include the papal arms of Pope Pius IX -- under whom a restoration of the basilica was undertaken.
Finally, I would note for our more archeologically interested readers, underneath the basilica are the foundational remnants of the old Roman era temples:
