STONEHENGE It's one of the most mysterious places on earth: a strange circle of oddly arranged stones, standing silently over the plains of England. Legends have linked Stonehenge to everyone from the pagan druids to the magician Merlin. More recently, science has determined that this enigmatic temple was actually built by early Britons some 4,500 years ago. Yet the question of how workers moved 40-ton stones over 20 miles of rolling hillside- and how they set them upright in perfect alignment at their sacred site- has been a matter of mere conjecture... until now. NOVA sends archeologist Julian Richards, engineer Mark Whitby, and stonemason Roger Hopkins (of This Old House) on a mission: to move, raise, and cap a Stonehenge-like structure, armed only with Stone Age tools. Will they and their band of volunteers succeed- using clever techniques such as a timber track for transport or a sliding counterweight for tipping a huge stone into its hole? "Archeology can answer some question," says Richards. "And this has answered some of the questions about the task. But there's always going to be a mystique about Stonehenge." COLOSSEUM 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.