The rope bridge was discovered by Jack, Sayid, Hurley, and Charlie during their search for Rousseau. The bridge seems to be used for crossing a gorge that appeared to be 50-70 feet in depth with a rocky bottom. The bridge was barely able to hold Hurley's weight as he crossed it, and it finally collapsed during Charlie's attempt to cross it. ("Numbers")
- The producers (truly) stated that they would never give story time to reveal the construction of this bridge. 
For fan theories about these unanswered questions, see: Rope bridge/Theories
- Who built this bridge, why, and when?