Petroleum has subterranean origins. The only way to know for sure whether petroleum exists in a certain place is to dig a well. Many petroleum wells will have to be dug, before you might finally strike petroleum. Petroleum engineers now use scientific methods to select promising sites to drill for oil (on land and at sea).
They employ sensitive instruments like the magnetometer, gravimeter and seismograph to find subterranean rock formations which might contain crude oil.
The uncertainty makes drilling for oil a risky process
Drilling is a difficult and complex process and is often risky. Some oil wells will go many miles underground before the engineers are able to reach the deposit. Many oil wells are located offshore and have to be dug from offshore platforms, which are attached to the ocean floor.
Usually, the high pressure in the oil well pushes the petroleum to the surface. When the pressure decreases (this happens after a substantial quantity of petroleum has been extracted) the crude oil has to be removed by pumping or injecting air, water, carbon dioxide and steam into the well to create artificial pressure. Many enhanced petroleum recovery methods have been invented which can increase the yield from a well.