Did they do it with mirrors? That's one of the more bizarre explanations NOVA looks into while trying to reconstruct the mysterious building techniques of the Inca, the remarkable civilization who ruled the Peruvian Andes 500 years ago. "The Incas hadno arch, no wheel, no codified mathematics," marvels one anthropologist. "They couldn't even write." So how did they erect their magnificent, earthquake- enduring citadels in the clouds- without mortar, iron tools, or draft animals, out of huge blocks of interlocking stone? Follow a diverse team of experts as they chip away at some theories, using only traditional tools available to the Incas centuries ago. Hundreds of villagers join in the effort to haul these unwieldy blocks, across water and steep terrain, armed with long ropes- and collective muscle power. The Incas also built breathtaking suspension bridges out of grass, an art still known to a handful of Andean villagers today. Watch how (in an astonishing three days!) blades of grass are harvested, twisted into strands, and braided into the cables of a 100-foot gossamer-like bridge, swaying high above the raging river- but sturdy enough for even the queasiest NOVA researcher to walk across.