Hunan, China

Tourists walk on the newly opened glass bridge, which offers visitos the amazing view of the "heaven-linking Avenue", the twisting mountain road from the foot of the Tianmen mountains to the Tianmen Cave, in Zhangjiajie scenic area in Hunan province on Aug 1, 2016. [Photo/IC]On a ledge hundreds of feet in the air, the best way to avoid anxiety can also be the hardest to stick with: Don't look down. As long as you keep your eye s up, you won't know how high you stand. In recent years, a trend of glass-bottomed structures has swept across the world.