Oil pipelines are frequently in the news these days because they can have a big effect on oil prices. For example, if a war or terrorist activity threatens to shut down an important pipeline, oil prices can rise. Oil pipelines can also make the news when they leak.

One of the best-known pipelines in the United States is the Trans Alaska Pipeline System. It stretches north-to-south for 800 miles across the state of Alaska. The oil comes out of the ground at the North Slope near Prudhoe Bay, flows down the pipeline to the port of Valdez, loads onto a supertanker, floats down to the west coast, unloads into another pipeline and makes its way to a refinery. The Alaska pipeline is needed because Prudhoe Bay freezes in the winter, while the port of Valdez is ice-free year round.

At its simplest level, you can think of a pipeline as a pipe and a pump. The pump sucks up oil from a storage tank and sends it down to the other end of the pipe. If the pipe is only a few miles long, this simple system works fine. In the early days of oil drilling and distribution, pipelines this simple were common. But modern oil pipelines like the Alaska pipeline are far more sophisticated.

For example, the Alaska pipeline carries approximately 40 million gallons of oil per day, and can approach 100 million gallons per day if necessary. To handle this volume of oil, the pipeline is four feet in diameter. As the oil flows, friction against the pipe wall slows it down. Therefore, approximately every 100 miles, a pumping station boosts the pressure. The oil pressure inside the pipeline is over 1,000 PSI. To handle that kind of pressure, the pipeline is made of steel that is more than three inches thick.

That still sounds pretty simple - you have a big pipe and some pumping stations. But then you have to consider expansion. The steel in the pipeline can potentially get as cold as minus 60 degrees F. Or, with a maximum load of high-pressure oil flowing, it can get as hot as 145 degrees F. That's a 205 degree temperature swing. Keeping in mind that steel contracts when it is cold and expands when it gets hot, you can see the problem. An 800 mile long steel pipeline can grow or shrink by more than a mile depending on the temperature. To handle that much expansion, the pipeline has to be able to expand and contract at all of its joints without leaking.

What happens if something goes wrong and the pipeline springs a leak? With the oil at such a high pressure, you can get a geyser. For example, in 2001, the pipeline was shot by a drunk man wielding a hunting rifle. Although the hole was small, more than 2 gallons of oil per second shot out of the pipeline. Since it took more than a day to find the leak, almost 300,000 gallons of oil inundated nearby trees and tundra.

Fixing the leak presents its own problems. You have to shut down the flow of oil on the entire pipeline, find the leak, isolate that section of the pipeline, drain the oil and fix the problem. It can be a huge, messy project. The Alaska pipeline has experienced hundreds of leaks over its lifetime.

You might be wondering how a rifle bullet could penetrate steel that is three inches thick. The problem is corrosion, which thins the steel over time and makes the pipeline more prone to leaks. To check for corrosion, pipeline engineers use a robot called a "smart pig" to run down the inside of the pipeline and check for problems. The thickness of the steel can be detected with a magnetic field. Where the steel gets thin, the way it handles a magnetic field changes, and sensors can detect those changes. When the pipeline gets too thin, sections have to be replaced.

Given all this complexity, you might be surprised to learn that there are oil and gasoline pipelines crisscrossing the United States. These pipelines stretch for tens of thousands of miles. They work around the clock to connect ports, oil fields, refineries, and major distribution points. Because these pipelines are almost always buried, they tend to be invisible. But they are essential to our day-to-day life. Without them, the flow of oil and gasoline in the United States would come to a standstill.

Marshall Brain is the founder of www.HowStuffWorks.com.