Aqueduto das Águas Livres

Lisbon's aqueduct spans the Alcântara valley. Construction began in 1731, and by 1748 the first water was flowing from a source 58 kilometres to the north-west. It bridges the valley on a series of 35 arches, the largest of which rises 64 metres from the ground; giddy heights indeed when you walk along the parapet. When built, these were the tallest stone arches in the world. They were sturdy too - they survived the 1755 Earthquake unscathed. The aqueduct was taken out of service in 1967. Guided walks, taking in some of the surrounding area, can be booked through the Museu da ígua.