From the World Heritage inscription:
Vizcaya Bridge straddles the mouth of the Ibaizabal estuary, west of Bilbao. It was designed by the Basque architect Alberto de Palacio and completed in 1893. The 45-m-high bridge with its span of 160 m, merges 19th-century ironworking traditions with the then new lightweight technology of twisted steel ropes. It was the first bridge in the world to carry people and traffic on a high suspended gondola and was used as a model for many similar bridges in Europe, Africa and the America only a few of which survive. With its innovative use of lightweight twisted steel cables, it is regarded as one of the outstanding architectural iron constructions of the Industrial Revolution.
I had no idea what this was until I visited Bilbao. I thought visiting a bridge would be sort of lame, but it turned out to be really interesting. The bridge is still functioning and carries cars and passengers every day. It can carry six cars every time the gondola goes across the river. If you visit, you can climb the bridge and cross it by foot on the top.
View my complete list of UNESCO World Heritage sites.