Drive past a water tower, and it appears to be a silent, passive giant. But it is the central player in a high-pressure balancing act.

Most municipalities obtain their water from a reservoir or well, purify it at a treatment plant and send it to a pump house that fills one or more elevated tanks. The pumps alone are strong enough to push water throughout a town's network of pipes, but the system's pressure--at your sink--would fluctuate as usage rose and fell and could drop too low to reach spigots during high demand. "A water tower acts like a capacitor. It maintains constant pressure on the lines and provides backup supply when demand exceeds pump output," explains Malcolm Jackson, a general manager for Utility Service Company, Inc., in Perry, Ga., which provides tank services nationwide.