Ponte da Arrábida

This guide to Ponte da Arrabida in Porto follows Edgar Cardoso, an engineer in his forties who staked his reputation on building two enormous concrete arms out over the Douro gorge from opposite banks, with nothing beneath them but open air. It's the late 1950s, and his method was untested at this scale: cantilever a little further each week and let the two halves close the gap over the water. Listen for the staircase climbing inside the arch above you.