Whatever was in this hole is now gone but we'll get to that later. But look below the street level and you see a small arch structure followed by a wall section followed by two more small arch structures -^-^^. Here's a photo, with the Temple of Saturn across the street in the background. That first arch is a just modern reconstruction using some fragments from the Arch of Tiberius that was located here. The brick space to the right has a small marble plaque saying 'Arco di Tibero'. And to the right of that are two arch structures. These two arch structures are two of the original eight arch structures.
The first one was where the modern reconstruction is, the second was where the brick space with the plaque is and then to the right of the two original ones are four more (these arches are excellent examples of opus reticulatum). These small arches make a small viaduct for the street above.
When the Temple of Saturn was rebuilt larger in 42 BC that street was farther back and had to be moved about 2 m closer to the Forum Square. So rather than wall and then fill this space with dirt they built this viaduct and put the street on top of it (the brick wall there today is modern). When the arch section (out of view) to the right was first discovered in the nineteenth Century they wrongly thought it was Julius Caesar's Rostra that had been built-over when Augustus' Rostra was built. Now the two missing original arches (where the reconstructed arch and wall with the plaque are) were demolished or built-over when the Arch of Tiberius was built in the area in front of them.
But first there *might* have been something else here before the Arch of Tiberius? It's possible that this was the location of the...Next: #14.3: Lacus Servilius