Umbilicus urbis Romae

The Umbilicus Urbis Romae—"Navel of the City of Rome"—was the symbolic centre of the city from which, and to which, all distances in Ancient Rome were measured. It was situated in the Roman Forum where its remnants can still be seen. These remains are located beside the Arch of Septimius Severus and the Vulcanal, behind the Rostra. Originally covered in marble, the Umbilicus is now a forlorn-looking brick core some 2 metres high and 4.45 metres in diameter.

Source: Wikipedia