Meta Sudans

You won't find it if you are in Rome today.  The remaining part of the crumbling structure was bulldozed on Mussolini's orders in 1936.

(Meta Sudans, in front of Colosseum, 1890, from flashbak)

The precise purpose of the Meta Sudans is still debated.  What is known is that it was built in the 1st century AD, during the same period as the Colosseum.  Nor does it have anything to do with the country of the Sudan.  It's meaning in Latin is roughly "sweating turning post" and it's thought it served as a fountain and point on which Roman Triumphs turned left, from heading along the valley between the Palatine and Caelian hills, and proceeded up and over the incline on which sat the Arch of Titus, and then on down into the Roman Forum.  Below is an 1860 photo showing the remnants of the Meta Sudans looking toward the Arch of Titus; an arch commemorating a dark time in Jewish history, Rome's suppression of the Jewish Revolt of 66-70 AD and the destruction of the Second Temple in Jerusalem.
(from roger pearse)

This is an artist's impression of what it may have looked like during the time of the Roman Empire.  Next to it is the Arch of Constantine, which was not constructed until around 320 AD.  Behind is a corner of the Palatine Hill, which held the Emperor's Palace.
 
(From detritus of empire)