What Sits Underneath St. Peter's Basilica in Rome

As you look down into the Confessio toward the Niche of the Pallia (where the pallium are kept), the floor level seen here is close to the floor level of Constantine's original basilica. 
Most are already more than familiar with the altar and baldachin of St. Peter's Basilica in Rome as well as the confessio that sits beneath of it. What they may not, however, be as familiar with is exactly just what sits beneath that, beyond what they can see with their own eyes. Today I thought it would be of interest to show a little bit of what, archeologically speaking, sits behind and beneath this, for there is a great deal more to it than meets the eye.

First, here is a cross-section view, taken from the south side of the present high altar, which may help to set some familiar context:

Here is a another view of the same:

The first thing one might note here are the locations of the various historical altars of the basilica. But before we get into those, if you were to step back further, one can see a little bit more context.

The altar marked with a "2" is, of course, the present high altar of St. Peter's Basilica.  You can also see the steps leading down to the confessio, leading toward the familiar Niche of the Pallia (which some might mistake for the tomb of St. Peter itself).

The section coloured red (and labelled #6) is what is now known as the Clementine chapel. Here is a photo of it as it appears today. As you can see above, this chapel sits directly beneath the steps that lead up to the present high altar of St. Peter's.

Now this red/pink section is on the level of today's papal grottoes, but perhaps unknown to a great many is that there is yet another level even beneath these papal tombs known as the Vatican necropolis. (See the blue coloured section above and below). These pre-date the time of the Emperor Constantine and it is at this level which the tomb of St. Peter may be found.

A drawing shows this ancient section (in blue) more clearly in its original appearance before the basilica was built on top it::

But returning to the present and historical altars of the basilica, here is a view of the layers of history that can be found beneath the present high altar:

In yellow is the present altar of the basilica
Shown above in red is part of the white marble wall (with a strip of red porphyry) that is the Emperor Constantine's own fourth century memorial for St. Peter. Here are two photographic views that show it -- the second as seen today through the grill of the altar of the Clementine chapel:

The rectangular shape seen here is a second, smaller altar that St. Gregory had constructed . It is situated opposite the altar of the Clementine chapel)
The altar of the Clementine chapel. The wall of the the Constantinian memoria can be seen behind the altar cross and candles.  Drectly above that was the location of the altar in the time of St. Gregory the Great. 
To give a better sense of what this was, the following is a model that recreates the entirety of the Constantinian memoria that can now only be seen in part:

Another angle to view the Constantinian memoria. You can see here the original Solomnic columns brought from Jerusalem which are now situated  high above the present high altar of the basilica on the mezzanine level.

Here too is a closer look at one of these original Solomnic columns that, at one time, surrounded the memoria of Constantine:

This original Constantinian arrangement was later modified by St. Gregory the Great, adding an altar overtop the original Constantinian memorial:

This arrangement would essentially stand until the erection of the present basilica.

Now all of this, I should note, is predated by the original grave and necropolis shrine which appeared as follows (the brownish-red coloured wall being what is referred to in these diagrams above as "the red wall").

A digital reconstruction he pre-Constantinian shrine of St. Peter in the Vatican necropolis. The open spot shown here is situated above the tomb of St. Peter approximately where the niche of the pallia is located in the present basilica.

All of this construction ultimately has the purpose of giving honour to the actual tomb and bones of St. Peter which can be seen here below,. The bones, seen here, are now protectively encased and were originally found wrapped in precious fabrics.

This, admittedly, can get a bit confusing, so if you wish to see a  bit more of this backstory that leads up to the original Constantinian Basilica of St. Peter, the following is a good video reconstruction:

After this and over top all of it, the original Constantinian Basilica of St. Peter's was erected:

After falling into disrepair, this would finally be replaced by the present, 16th century basilica of St. Peter's that we all know and love:

The dome of the new St. Peter's seen rising above the old Constantinan basilica.

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