The Meta Sudans was a fountain in Rome on the Appian Way, just inside the Arch of Constantine. Its remains were demolished by Mussolini to make way for a road. In old photographs it is usually photographed from the Arch of Titus, which makes it look more complete than it was. Today I found online another photograph from the other side here.
Today only foundations remain. I took care to look for them on my recent visit to Rome. There is a circle in the grassy area in front of the Arch of Constantine.