The Aqueduto das Aguas Livres is Lisbon's great 18th-century aqueduct, built to bring water into the city from the hills northwest of town. Its best-known section strides across the Alcantara valley on tall stone arches, an impressive piece of engineering as well as a city landmark.
Parts of the old water system can be visited through the Water Museum, including the Mae d'Agua reservoir at Amoreiras. The aqueduct is a good stop for visitors interested in the infrastructure that shaped historic Lisbon.