Right in the center of Sofia, beneath the modern streets and metro station, lie the remarkably preserved ruins of the Roman city of Serdica. Emperor Constantine the Great famously said "Serdica is my Rome." The ruins include ancient streets, buildings, baths, and early Christian churches, all visible through glass floors and open-air displays.
Visible 24/7 through glass floors at the metro station. Free to explore. The best-preserved sections are between the Presidency and the Sheraton Hotel.