More than 2,000 years ago, Rome became the first city in the world with a population of 1 million people. And one of the reasons? Abundant, clean water.

Rome’s earliest settlements go back 10,000 years. The famous “Seven Hills of Rome” were first ancient hilltop villages that gradually merged into one city.

Rome’s early citizens took their water from the Tiber River, but the growing population required more. So, Rome’s early engineers went looking for it.

And engineers were required, because Rome set out to build aqueducts. The first was just 10 miles long, tying together a series of springs. They buried its entire length for defensive reasons.

With that success, Rome built 10 more aqueducts, stretching as far as 60 miles from the city. Many were also underground. But some had to be carried on long, arched colonnades to cross low points in the landscape.

To keep the water flowing but the water pressure manageable, the aqueducts were carefully constructed with very shallow slopes, sometimes as little as a tenth of a percent.

At their peak, Roman aqueducts brought in nearly 40 million gallons of fresh water each day, supplying 900 bathhouses and 1,200 fountains.

2,000 years later, the famous Trevi Fountain is still fed by one of them.

If you have a chance to see it, you’re experiencing not just beautiful art, but an engineering marvel of the ancient world.