The major construction project completed in ancient Rome around 80 AD was the Colosseum, also known as the Flavian Amphitheater. This monumental structure was commissioned by Emperor Vespasian and completed under his son Titus. The Colosseum could hold up to 80,000 spectators and hosted various events, including gladiatorial contests, animal fights, and public spectacles. It remains a symbol of the grandeur of Rome and its engineering prowess, showcasing the importance of entertainment and social interaction in ancient Roman culture.