Umbilicus urbis Romae

Roman legend related that Romulus, when he founded the city, had a circular pit dug in the Forum.

The first fruits of the year were thrown into this pit as a sacrifice and all new citizens of Rome had to throw in a handful of dirt from their place of origin.

[1] The Mundus (Latin, "world"), known only from literary sources, was an underground structure considered a gate to the underworld.

These days were considered dies nefasti—days on which official transactions were forbidden on religious grounds—because evil spirits of the underworld were thought to escape then.

The Umbilicus is believed to be a separate structure from the Milliarium Aureum, which was built nearby by Augustus (c. 20 BC) and served much the same purpose for distance reference.