Earlier this month, the officials of China's Hunan Province revealed what they believe will be a new "Wonder of the World" - the planet's longest and highest glass-bottom bridge. When complete, the 430m (1,214 ft) long and 6m wide structure will connect the two summits across the Zhangjiajie Grand Canyon. While the length is certainly impressive, it is the 400 m (1,312 ft) height that will have most visitors gripped by fear as they try to traverse across.