While fossil fuels have only become the driving force behind human civilization in the last couple of centuries, oil and natural gas have been making their way back to the Earth's surface for millions of years. Spanish conquistadors observed oil rising to the surface in the Gulf of Mexico in the 16th century, and the Chinese drilled for it in the ground as early as 347 A.D. [source: Totten]. To find even older evidence, you don't have to look any farther than the prehistoric animals unlucky enough to have been consumed by the world's tar pits.
However, most of the world's petroleum is trapped between 500 and 25,000 feet (152 and 7,620 meters) under dirt and rock. All of this oil began as tiny plants and animals called plankton, which died in the ancient seas between 10 and 600 million years ago. This decaying matter drifted to the bottom of the ocean and, over time, was covered with sand and mud. In this oxygen-free environment, a kind of slow cooking process took place. Millions of years of heat and pressure eventually transformed this organic material into vast deposits of liquid, gas and solid petroleum, all capped in traps under thick layers of rock. We call liquid petroleum oil and gaseous petroleum natural gas. Solid petroleum deposits often take the form of oil shale or tar sands.
Needless to say, these fossil fuel deposits don't just start bubbling crude every time a hillbilly fires a rifle. Geologists study surface features and satellite maps, check soil and rock samples, and even use a device called a gravity meter to find subtle gravitational fluctuations that might indicate a subterranean flow of oil. Not all of these options are particularly viable, however, if the terrain you're canvassing is thousands of feet below pitching ocean waves.
When searching for fossil fuels at sea, oil geologists are able to use special sniffer equipment to detect traces of natural gas in seawater. But as this method can only help find seeping deposits, oil companies largely depend on two other means of locating traps.
When close to the surface, certain rocks affect the Earth's normal magnetic field. By using sensitive magnetic survey equipment, a ship can pass over an area and map any magnetic anomalies that occur. These readings allow geologists to hunt for the telltale signs of underground traps.
Surveyors can also detect possible traps through the use of seismic surveying. This method, known as sparking, involves sending shock waves down through the water and into the ocean floor. Sound travels at different speeds through different types of rock. If the shock wave reaches a change in rock layers, it bounces back up toward hydrophones dragged behind the survey ship. With the aid of computers, seismologists can then analyze the information to pinpoint possible traps in the Earth.
Survey ships use both compressed air guns and explosives to emit shock waves. Of these two methods, air guns are far less of a threat to sea life, but even acoustic pollution poses a threat to such seismically aware sea animals as the endangered blue whale.
What happens once survey teams detect undersea oil deposits? Well, it's time to mark down the GPS coordinates, plant a buoy and obtain a government lease to begin a little exploratory drilling and see what you've got.