Umbilicus urbis Romae was the symbolic centre of the city between the Arch of Septimius Severus and the Vulcanal. According to the legend there was gate to the underworld - Mundus and Umbilicus urbis Romae could be a part of the subterranean Mundus.
See:
- https://en.wikipedia.org/wiki/Umbilicus_urbis_Romae
- Gilbert J. Gorski; James E. Packer. The Roman Forum: A Reconstruction and Architectural Guide. Cambridge University Press. 2015, p. 155.
