Contrary to popular belief, more of the Circus Maximus survives than one might think. The "floor" level of the race track is now about 10 meters below ground level, and has never been systematically excavated. The spina (the central median of the track) was partially dug in 1587 on the orders of Pope Sixtus V. They were looking for the great Egyptian obelisks which had once adorned it, and found two of them: the bronze age granite obelisk taken by Augustus from Heliopolis, which now stands in the Piazza del Populo; and the massive, 522-ton obelisk originally quarried under Thutmosis III (1500-1450 BCE) for the temple of Amon at Karnak, and now standing in the Piazza S. Giovanni in Laterano. In modern times, the starting gates at the west end were briefly excavated (1908) and then reburied.

There have been two major excavations of the still-extant seating on the east end (1930 and again 1979-88), which revealed a confusing series of Trajanic and later rebuildings. It was, apparently, a structure which underwent frequent renovation. From those excavations it also seems apparent that the vast majority of the highest banks of seats, which remained above ground level, were robbed out in the early Medieval period and taken elsewhere, probably for building material. Seats for 250,000 spectators' backsides is a lot of marble, after all.

The last races were held in 549 CE by the Ostrogoth Totila. Thereafter, the structure seems to have fallen quickly into disuse. In the early Medieval period, the area was used as grazing and farmland, and the seating structures were converted into a variety of industrial works. The whole area was cleared out and put in its present, relatively cleared state in the early 20th century.

Why hasn't Italy excavated more of the Circus? First, there is not much new to discover there which is easily accessible. Second, money, which Italy does not have much of these days, especially for archaeology. Third, the space is popular and frequently used (I saw Sting perform there in 2004).

Why do some structures survive so well, while others virtually disappear? The christian church played a big role in the preservation or destruction of many ancient monuments. If it became sacred to the christians, it generally survived and was maintained; if not, then most places lacked the will or the funds to upkeep large and useless old pagan edifices. The local attitude towards preservation, for whatever reason, also played a role, as well as their access to sufficient building materials.

Unfortunately, two of the best and most recent sources are not in English. One is Marcattili, Circo Massimo : architetture, funzioni, culti, ideologia (Roma : L'Erma di Bretschneider, 2009); the other is Polt, Circus Maximus : das gesammelte Werk ; Geschichten, Stücke, Monologe und Dialoge (Zürich : Kein & Aber, 2002).