The 109 arches of the Aqueduto das 脕guas Livres lope across the hills into Lisbon from Cane莽as, more than 18km away; they are most spectacular at Campolide, where the tallest arch is an incredible 65m high. Built between 1731 and 1799, by order of Dom Jo茫o V, the aqueduct brought Lisbon its first clean drinking water. Its more sinister claim to fame is as the site where 19th-century mass-murderer Diogo Alves pushed his victims over the edge.
Another of the best places to see the aqueduct is in the leafy Pra莽a das Amoreiras next to the M茫e d'脕gua.
Contact Lisbon's Museu de Agua (details are on the EPAL website) for guided tours and information in season.