The ancient Romans are renowned for their impressive aqueducts, which supplied water to their cities and towns. Some of these aqueducts stretched over long distances—like the aqueduct of Valens, which transported water to Constantinople (modern-day Istanbul) and was nearly 250 miles (400 kilometers) long. Today, the remnants of these aqueducts remain some of the most notable examples of Roman engineering, standing as lasting testaments to their skill and innovation. To build these aqueducts, the Romans first needed to find a reliable water source, often choosing natural springs. The aqueducts were designed with a very slight downward slope, allowing water to flow naturally from the source to the city. This careful engineering ensured a continuous and efficient water supply. Roman surveyors used tools like the dioptra, a disc-like instrument with a sighting bar, and the libra, which featured a plumb bob, to measure ground elevations and determine the ideal route for the aqueducts. These tools helped them maintain the precise gradient necessary for water flow. The durability of Roman aqueducts can be attributed in part to the special concrete they used. This concrete, which included pozzolan—a type of volcanic ash—combined with lime and water, became extremely strong when exposed to water. This material was particularly useful in constructing bridges, arches, and other key structural components. However, the longevity of these aqueducts wasn’t solely due to their concrete. The Romans also chose stable geological areas for their aqueducts, avoiding unstable landscapes like active volcanoes. Additionally, regular maintenance, which continued well into the Middle Ages, helped preserve these structures over centuries. Not all aqueducts were built above ground. In areas where the terrain made it difficult, parts of the aqueducts ran underground. As the water neared the city, it would often pass through basins that helped filter out dirt and debris. Some aqueducts had only short above-ground sections, while others featured long rows of arches. The size and complexity of an aqueduct also determined the number of workers required to build and maintain it. For example, the Hadrianic aqueduct in Athens, which is about 12.4 miles (20 km) long, could have been completed by around 500 workers in 15 years. Even after the fall of the Roman Empire, many aqueducts continued to be used and maintained. Some were in operation well into the Middle Ages and even today. One of the most famous examples is the Aqua Virgo, built in 19 B.C., which still supplies water to Rome. The continued use and upkeep of these structures highlight the enduring legacy of Roman engineering.