The Stadium of Domitian (Stadio di Domiziano) was built in 80 AD as a venue for sporting events and was the first of its kind. It was designed in an elongated U-shape using a Greek architectural style, and it could hold between 15,000 and 20,000 people. In the 15th century, the stadium was paved over to make Piazza Navona, which is a popular square in Rome today. The square is almost the exact same size and shape as the stadium was. Some pieces of the original stadium can still be seen above ground, but much of the remains are underground.
Today visitors can see parts of the archaeological site and get a glimpse of Rome's past. Graphs, photos, and videos explain the history of sport as well as the history of the stadium itself. Much of what lies underground is off limits and can only be accessed with special permission, but the parts that are visible are spectacular. The stadium is one of many examples of the many layers of history that exists in Rome.
Stadium of Domitian is located at Piazza Navona. Several buses stop at Piazza Navona, or it's a 15 minute walk from Trevi Fountain. Opening hours are 10am to 7pm Sunday to Friday, and 10am to 8pm on Saturdays. Admission is 8 euros.