• 1. In ancient times, oil was collected at the surface.

• 2. In the 19th century, holes were drilled to depths of several dozen meters.

• 3. Today, drillholes reach as far down as several thousand meters.

From the time of high antiquity, in ancient Mesopotamia, oil that had seeped to the surface was collected for medicinal use, as well as lighting fuel and caulking for boats. Today, now that we have been producing from accessible reservoirs for 150 years, it is increasingly hard to find hydrocarbon-impregnated rock. Explorers now have to look hundreds and even thousands of meters below ground.