Roughly 2,100 years ago, Roman engineers constructed this architectural marvel—a 15km (9 1/3-mile) conduit to bring water from the Guadarrama mountains to Segovia. The graceful feat of engineering remains as impressive as it was in the age of the Caesars. While much of the original aqueduct was a ground-level canal, the concluding segments arch high over the city and then continue underground all the way to the Alcázar. The entire structure was built of granite blocks without mortar. Following restorations in the 15th and 16th centuries, it continued to supply the city’s water into the late 19th century. The highest of the 166 arches is 28m (92 ft.), and seems even higher when you stand under it and look up.