Researchers have unearthed a colosseum in Tuscany that seated as many as 10,000 people. It was built, the suspect in the 1st century. Strangely no historical record of the structure exists. It would have been about a fifth the size of the colosseum in Rome.
This is how civilizations vanish. At some point the local authorities, probably in the 4th century (if not earlier) could not afford to pay for its upkeep. Private contractors probably used it for one thing or the other but they didn't spend any funds for upkeep, so it continued to decay. People used it for illicit meetings, homeless and other wanderers gathered there and wore its structure away further. Grass grew between rocks and gave room for other debris that worsened it. Eventually deadfall built up and slowly the whole structure vanished beneath the loam.
Until at last some old dude was standing over it, talking to his grandchild and saying..."right here, there was a colosseum! it held thousands!" and the kid said "yeah, whatever pops. colosseum. right here. sure."