The author reports on late 19th-century excavations that revealed extensive substructures beneath the arena, showing earlier origins than commonly supposed and multiple construction phases from Scaurus and Nero to the Flavian emperors. The work describes underground features such as a movable boarded arena with corbels, animal cages with vertical lifts and trapdoors, canals and reservoirs for staged naval displays, and wide passages for scenery, and examines reused timber and stone, masonry of tufa, brick, and concrete, plus coins and graffiti as documentary evidence. It also traces repairs from earthquakes and argues the amphitheatre evolved over more than a century rather than being completed in ten years.