The Salaborsa is Bologna's main public library and one of the city's most distinctive interior spaces. The central hall — once the trading floor of the Bologna stock exchange — has a glass floor that allows you to look down at the Roman ruins below: the basilica and forum of Bononia, the city founded by the Romans in 189 BC.
The space is heavily used by Bolognese students and free for anyone to enter. Wi-Fi, comfortable seats, and good light make it one of the better places to spend an hour out of the rain. The glass floor is open even when the library is closed if you walk through the courtyard from Palazzo d'Accursio.