Roman cities wouldn't have thrived the way they did if not for a very sophisticated system of aqueducts that delivered water for residents' personal consumption, as well as for their public baths and fountains. Cities situated on dry plains generally relied on faraway mountain runoff as a source of clean water. But even cities built near rivers often used remote water sources instead of local ones, which commonly became polluted by the waste generated by the cities' dense populations.

Aqueducts were by and large gravity systems. In other words, they were designed to use the force of gravity to move the water, frequently across tens of miles. The channel through which the water flowed had to slope downward ever so slightly, which meant the engineers -- always keeping in mind the diverse terrain that might lie between source and destination -- had to build whatever solutions were necessary to achieve this feat.

Because some natural springs ran from underground sources, and because pumping water to the surface was not easily accomplished, some aqueducts began as tunnels, dug through the earth and even through rock right to the source. Arcades, many of which still line the landscape over regions that once belonged to the Roman Empire, served as bridges so that water could travel over land and span any dips in it. The arcades' design allowed people and water to pass beneath them. Arcades were constructed when the aqueduct had to flow five feet or more above ground level; any lower than that, and builders would have built a simple wall. Where the aqueduct's intended path ran into mounds of earth, trenches were cut through them to preserve the downward slope.

Once the water reached a city, it was typically directed into a large water-storage tower positioned on high ground. From there, water could be distributed through an underground network of lead, wood, or terra-cotta pipes to various points. The Romans knew that, even though the water would run underground through the pipes, it could be brought up again to a height equal to the water level in the tank.