Going underground in Rome or Naples is quite the experience. But Cagliari, Sardinia, has a compelling underground experience as well. Look for street signs with a map labeled “itinerario cultural pedonale” which will take you to the historic churches and the crypts that sit below them. But if you are looking for Roman era remains on your underground tour, you need to go to the Museo del Tesoro e Area archeologica di Sant’Eulalia—and what you want is the “area archeologica” if you want to get below the level of the modern city, where a whole hidden city awaits.
The excavations below the museum took place in 1990 as the museum was being worked on, and you’ll see Roman roads, house remains, and even a Roman Toilet from the Roman Republican era:

In the museum above you’ll find historic objects from several churches, some of which were bombed during WWII, and you’ll find artistic and archeological works on the first floor.
Museo del Tesoro e Area archeologica di Sant’Eulalia is located on vico del Colleggio 2 in the Marina neighborhood of Cagliari, conveniently located near the Port.
More Archaeology in Sardinia
Nora: Guide to the Ancient City in Sardinia