Museum Aqueduto das Águas Livres in Lisbon

You're standing by the Águas Livres Aqueduct, the aqueduct of the free waters. Look at the long row of arches striding across the Alcântara valley. There are thirty-five of them, almost a kilometre end to end. The tallest is the Arco Grande, sixty-five metres high. It's one of the tallest pointed stone arches anywhere in the world. Notice how some arches are round and others come to a point? The pointed shape was chosen because the arches here had to be so tall and so wide.