Ancient Romans flocked to this massive amphitheater by the tens of thousands, eagerly awaiting a bloody spectacle of man or beast. As each gory show played on, the audience was sure to stay cool and calm- thanks to a billowing canopy that protected them from the blazing Mediterranean sun. Without modern materials how did the Roman builders complete the awesome task of covering the Colosseum? Even today's engineers don't usually attempt to roof an entire arena- and those that do (like at the Houston Astrodome) rely on steel supports and lightweight plastic fiber. NOVA tries out two competing concepts: One, a traditional theory, holds the a giant spiderweb of ropes secured reams of cloth covering- a notion that has never actually been tried (until now). The other- a new hypothesis conjured up by historian Rainer Graefe borrows from the elegant mast and boom construction that was used to unfurl the canvas sails of ancient ships. Who is right? Not a shred of the roof has survived, and so every clue is precious- whether below the streets of Rome or on a puzzling painting from Pompeii.