Diesel comes from a variety of places around the world, from places such as the USA , China, Saudi Arabia and Norway - not just your local fuel garage!
Diesel is a product of the fractional distillation of crude oil, mined in places all over the world. Fractional distillation occurs in a fractioning column. Crude oil is a mixture of different length hydrocarbons, which when boiled at temperatures up to 400 degrees celsius, separate into different fuels, one of which is diesel. The hottest point of the fractioning column is at the bottom, so the lighter fractions (different products) come out first. Diesel leaves the fractioning column at around 300 degrees celsius.
In 2005, the USA was the worlds largest producer of diesel, producing roughly 18.6% of overall global production; the second biggest was China at roughly 10.2% of overall global production - quite a substantial difference!
Seismic waves are used to detect oil reservoirs in the Earth, often caused by manmade explosions (hopefully far away from where you live!)
To then extract the crude oil from underground, wells must be drilled into the reservoir. When the well reaches the oil, it will be recorded by a present geologist, known as a 'mudlogger'. Oil wells are created by drilling very big, very deep holes into the earth above the oil reserve using an oil rig. The oil is then extracted via his hole, using a collection of valves which control the flow of the oil, whilst regulating the pressure inside the chamber.
In the past, particularly in the USA, there were sites known as oil fields where the oil would naturally rise to the surface. These sites have since been completely used up, except for specific places in Alaska.