When the Romans designed the city of Bologna in the 2nd and 3rd century BC,they really built it to last. And last it did: Excavation of the historic center of Bologna in the 1990s uncovered the two main streets built during the Roman Empire, still intact beneath the gorgeous Salaborsa Library in the city's most common piazza.
The Salaborsa Library (Biblioteca Salaborsa) is a contemporary space housed within a beautiful historic building, and sitting on top of an extraordinary piece of history. You can see the underground ruins from the main room in the library,through the clear glass panes in the floor. It's worth getting a closer sight though, so head downstairs to the small museum to further explore this hidden remnant of antiquity.
Before you enter the archaeological area, or there is a timeline of the history of Bologna,at the cessation of which is a door. Through the door, you are transported back in time, and surrounded by ancient building foundations,a well, and even an ancient sewage system. A kind Italian guide may even be available to point out the different layers of history, or which are clearly visible in the remarkably preserved structures. The deepest,oldest levels of the ruins may date back even earlier than the founding of Bologna, suggesting a settlement already existed here before the Romans.
Source: atlasobscura.com