Aqueduto das Águas Livres - Aquaduct of Free Waters – what a great name for a city's water supply. However, its construction in the 18th century was financed by additional taxes, including taxes on olive oil and wine. The result is impressive, as you can see when approaching Lisbon airport from the southwest. To your left, you can spot the centrepiece of the aqueduct, the section that spans the Alcantara Valley (a few minutes earlier you have a nice view of the Belem WHS). But the Águas Livres Aqueduct is more than that, the entire canal system has a length of 58 kilometres.

The main section of the aqueduct begins in Belas, about halfway between Lisbon Airport and Sintra. Here, in Mãe d'Água Velha (photo), water from the springs was collected via a system of canals and further transported to Lisbon. The walled water pipeline and the air shafts are visible. Between Caneças and Belas, a visible part of the aqueduct runs a few kilometres along the N250 (or rather the N250 runs along the aqueduct). It was the first stop on my Portugal trip in May 2018 and only the first part of my visit to the Águas Livres Aqueduct.
The Alcantara Aqueduct is the main point of interest of this TWHS. Without a doubt, it is impressive to look up from the bottom of the valley to the top of the pointed arches, the highest being 65 metres high. But the walk at the top of the aqueduct is a little disappointing. In the 18th century, the view over the Alcantara Valley may have been picturesque, but today a six-lane motorway, roads and railway lines run under the arches. Nothing exciting in a city with an abundance of spectacular viewpoints. You can walk the entire distance of 941 metres - and back. The only entrance is on the Amoreiras side. And you can take a look inside the water pipe when changing from one side to the other about halfway. 
Finally, I also visited the end of the aqueduct, the Mãe d'Água das Amoreiras, where the incoming water was stored and distributed throughout Lisbon. From the outside, the reservoir is just a simple block. But the inside is quite nice, a high room with a vaulted ceiling supported by four massive columns, a bit like a cathedral. The basin is still filled with water, but the entire aquaduct and also the reservoir was closed in 1968. Today it is used for events and exhibitions, and it has regular opening times. It's worth a detour if you are going to the Alcantara Aquaduct anyway. The staff will be happy, usually there are not many visitors.

Aquaducts are already well represented on the WH list. Although most of them date from ancient times and are only preserved as ruins, I don't think more are required.
It is claimed on the Unesco website that the Águas Livres Aqueduct "was the last great classical aqueduct to be built all over the world". I am not sure if this is true, but it is a rarely used superlative among World Heritage candidates. More often the nominations are promoted as the first in a particular category. A big plus for Águas Livres is that the entire water supply system has been preserved, it was in operation until the late 20th century. Nominations on the topic water management have been very successful at the WHC in recent years, so I would not be surprised to find the Águas Livres Aquaduct also on the WH list in the near future.