The first Roman Aqueduct was constructed in 312BC, when Roman was becoming a Republic. This Aqueduct was named the Aqua Appia. The Aqua Appia was constructed using roman concrete, and set stone, but the Roman's had a problem. So much materials would have to be gathered to make an Aqueduct of this size. They had to design a new architectural method to save them materials, be strong enough to withstand the forces of nature, and mankind. The method they came up with, was the famous Arch.
The arch was a big success for the Romans, it had great strength, and saved them materials. To construct the arch, they used wooden beams, and created a frame. After creating the frame, they put the frame into place, and put the stones on top of the frame. Then once they got to the last, top, piece, they had to place a keystone. The keystone had to be crafted perfectly to fit the two sides. If they keystone wasn't crafted right, the whole arch would not be sturdy, and will most likely fall.
Some of these Aqueducts would go for miles, and miles. In total, the Romans had a combined length of at least 500 miles of Aqueducts. The longest one, Constantinople, was at a length of 57.5 miles. To protect the aqueducts, only 5.8% of them were above ground, the other 470 miles were underground. Keeping the aqueducts underground, would protect them from disease, and protect them from enemy attack.