Well ya can't miss it, it's that ugly brick circular structure next to the Arch and directly in front of the Altar of Saturn and signposted 'Umbilicus Urbis Romae'.
It is actually two separate but interconnected sites, one above the other.
The 'Ubilicus Urbis Romae' is the brick structure you can see which are the remains of the 'symbolic center' or navel/bellybutton (umbilicus) of the City of Rome and the entire Roman Empire. Just like the Omphalos in Delphi was the ancient navel/center of the World in its' heyday.
Below that brick structure is the MUNDUS which is a hole in the ground (said to be a deep cleft in the ground) which connects (gateway) the living World above to the underworld of the dead (like ghosts and spirits, "inhabited by infernal beings and the shades of the dead"). Now the original Mundus started as a trench according to Plutarch. "Romulus buried Remus together with his foster-fathers in the Remonia and then set himself to building his city. After summoning from Tuscany (Etruscan) men who prescribed all the details in accordance with certain sacred ordinances and writings, and taught them to him as in religious rite. A circular trench was dug around what is now the Comitium and in this were deposited the first-fruits of all things the use of which was sanctioned by custom as good and by nature as necessary. And finally every man brought a small portion of the soil of his native land and these were cast in among the first-fruits and mingled with them. They call this trench as they do the Heavens by the name of "Mundus." Then, taking this as a center, they marked out the city in a circle round it. And the founder (Romulus) having shod a plough with a brazen ploughshare and having yoked to it a bull and a cow, himself drove it". Ok it's a cool ritual the first time it's done but must get kind-of old after a while :-) .
*Perhaps*? it's later decided to have a symbolic Mundus to throw in the first fruits and some hometown dirt. And a really great spot for it (if this is the original location?) would be on this higher ground natural speaking platform right, a really good spot to do this kind of religious ritual before the people. And it just happens to be in front of the Altar of Saturn? And then over the centuries its original meaning and function are lost and it becomes a gateway to the Underworld? Anyway in our timeline that is what this Mundus is, a gateway to the Underworld.
And three times a year (Aug24, Oct5, Nov8) the Lapis Manalis (stone lid) is removed and through this opening in the ground and out through that small doorway you see the denizens of the Underworld are let out to roam freely about the living World above.
And on these three days among other things there can be no battles or military operations, ships can't weigh anchor, no judgments, no public meetings, no marriages and if *you* are in Rome on those dates...No Sex is allowed either ("Not tonight Dear it's one of the Nefasti Dies (unlucky days)" :-) .
Ok this reconstructed UMBILICUS URBIS ROMAE is from the Septimius Severus era (about 200 AD) but the original probably dates to the second Century BC. The construction of the Arch of Septimius Severus encroached on the ancient Umbilicus, which was reconstructed to give space to the arch. Fragments of the older monument were used in the new one (they're the marble remains).
Some info claims that Severus moved the Umbilicus here to make way for his Arch but I don't think so.
Ok you be the judge of this. This monument *is physically* incorporated into the rear corner steps of Caesar's Rostra built about 260 years earlier. A short time later Augustus puts the 'Miliarium Aureum' at the opposite end of the steps which balances it out architecturally o=====o. I think there would have been some kind of major public and religious outcry over moving it, so loud that it would probably would have survived in writing today. It's not like just moving a building or statue it is after all an ancient gateway to the Underworld and the ancient center of the Roman Empire.
I think the Arch of Septimius Severus just encroached a bit on the original Umbilicus and it was rebuilt but still in it's original location. Or perhaps it just needed a facelift and got one to compliment Severus' Arch?
Now this Umbilicus has a cylindrical concrete core that was brick faced and built in three stacked sections. Its base is about 4.6 m and the top is about 3 m and it was faced in either white or colored marble. The top perhaps held either a statue of column.
Still standing wherever you are look at the back of the Rostra, that is the...
For more information and photos, please see Umbilicus Urbis in A Tourist in Rome.Next: #11.4: Rear Section of the Caesar Rostra