Ponte da Arrábida

This concrete arch bridge, opened in 1963 to carry the coastal motorway across the mouth of the Douro, was at completion the longest reinforced-concrete arch in the world. Its single graceful span leaps between steep granite banks near the river's outlet to the Atlantic, a feat of engineering by the Portuguese designer Edgar Cardoso. Now a listed monument, the structure can be climbed on guided bridge-walk tours that ascend the arch itself, giving vertiginous views along the river toward the historic centre and out to sea.