Lanciani had only found half of the structure, leading him to misinterpret its shape and function," said Simon Keay, project director and leading expert in Roman archaeology at the University of Southampton.
Measuring 138 feet by 125 feet (42 meters by 38 meters), the arena was found inside an imposing imperial-style palace.
According to Keay, the amphitheater's luxurious materials and its location, tucked away at the eastern end of the palace, suggest that the building was used as a private venue by high status officials, or even emperors.
It is likely that the emperors stayed at the palace before and after their travels, and possibly received distinguished visitors at the arena.
Keay estimates the structure could hold up to 2,000 people -- by comparison, the Colosseum could seat more than 50,000 spectators during public games -- but is cautious about its real function.
"It is unusual to find this type of building so close to a harbor, and it is unclear exactly what it was used for. Gladiator fights may have taken place there, as well as wild beast baiting, and even the staging of mock sea battles. It might have also been a form of Roman 'folly,' shaped like an amphitheater, but used as a monumental garden," Keay said.
Led by the University of Southampton, who worked with colleagues from the Italian Archaeological Superintendency for Ostia, Cambridge University and the British School at Rome, the three-year dig has revealed many other findings, confirming that Portus was of the largest maritime infrastructures of the ancient world.
"The project is the first ever large-scale dig at this unique site and mixes excavation, 3-D geophysics, computer visualization, environmental analysis and digital recording. This allows us to experience, analyze and interpret Portus in a wholly new way," Graeme Earl, one of the team leaders, told Discovery News.
With the help of ground penetrating radar, the archaeologists have uncovered luxuriously decorated rooms, a colonnaded garden, a finely carved marble head, possibly depicting the Greek hero Ulysses, and a well-preserved toilet, designed to be used by three people at a time.
"The toilet belonged to the palace. It is located between the amphitheater and a porticoed garden. It is really an impressive building, with marbled floor and walls," Keay told Discovery News.
The researchers are now analyzing the dirt from the toilet -- basically ancient human waste -- to build a picture of the diet of the people who frequented the site.
Another finding, a 295-foot (90-meter) shipping canal, further confirmed the importance of Portus.
Begun by the emperor Claudius in 42 A.D., inaugurated by Nero and greatly enlarged by Trajan in the 2nd century A.D., the harbor fed a city of more than 1 million down into the Byzantine period and beyond.
Ships from Egypt, Turkey, Greece and the Middle East unloaded supplies into 300-meter-long warehouses supported by 15-meter brick arches.
The goods, ranging from wheat and wine to slaves and exotic animals, were then transferred by canal to the Tiber for transport to Rome.
"This is one of the most important archaeological sites in the world. ... Certainly it should be rated alongside such wonders as Stonehenge and Angkor Wat in Cambodia," Keays said.
http://news.discovery.com/archaeology/mini-colosseum-unearthed-rome.html