IV century roman mosaics inside the Mausoleum of Santa Costanza, monumental complex of St. Agnes Outside the Walls, Rome
Built by Costantina, daughter of Costantin I, as a mausoleum near the basilica erected above the catacombs of St. Agnes , it would later become known as the church of Santa Costanza, when Constantina was venerated as saint.
Catacombs also live nearby. Good luck getting them to let you in, though.
(Source: irefiordiligi, via ancientromebuildings)