GENZANO, ITALY—Archaeologists excavating thermal baths at the site of a villa southeast of Rome believe they've stumbled onto a "mini-Colosseum" that was likely erected by Emperor Commodus, best known as the tyrant portrayed by Joaquin Phoenix in the movie Gladiator. The elliptical structure dates to the A.D. second century, is 200 feet by 130 feet, and would have held 1,300 people. The research team believes this was the venue where Commodus, who ruled Rome from A.D. 180 to 192, practiced for his gladiatorial performances that earned him the nickname "the Roman Hercules."