PCOL writes "When Conquistadors came to Peru from Spain in 1532, they were astonished to see Inca suspension bridges achieve clear spans of at least 150 feet at a time when the longest Roman bridge in Spain had a maximum span of 95 feet. The bridges swayed under the weight of traffic terrifying the Spanish and their horses, even though, as one Spaniard observed, they were almost as "sturdy as the street of Seville." To build the bridges, thick cables were pulled across a river with small ropes and attached to stone abutments on each side. Three of the big cables served as the floor of the bridge, two others served as handrails and pieces of wood were tied to the cable floor before the floor was strewn with branches to give firm footing for beasts of burden. Earlier this year students at MIT built a 70-foot fiber bridge in the style of the Incan Empire. The project used sisal twine from the Yucatan Peninsula and anchored it by wrapping it around massive concrete blocks. The weekend's burst of activity was preceded by 360 hours of rope-twisting as the 50 miles of sisal twine was turned into rope. Working together as a group was part of the exercise. "A third of the time was spent learning to work together," one of the students said. "But after a while, we were banging those cables out.""