The “Hall of the Lost City” focuses on the parts of the city that have been lost in the course of modern excavation activities. Due to the sometimes relentless measures since the days of Napoleon, medieval and modern quarters have now also become the subject of historical research. From this corner of the building, visitors have a particularly good view of the former "Quartiere Alessandrino", which had to give way in Mussolini's time in favor of the great parade street
(today Via dei Fori Imperiali) and the adjacent excavations. A model in the middle of the hall shows the once dense district, in the place of which a downright fallow area has moved. In the distance, archaeologists are breaking off one of the last sections of the old Via Alessandrina to make the Trajan's Forum below become more visible (Even "new" old parts will be
). Even today the city is constantly changing. The question arises as to which past has the higher priority. Above the photographs on the wall, the hall disappears into seemingly infinite darkness.