But if oil comes from ocean dwelling organisms, you may ask, why is so much of it concentrated in the dry, arid Middle East? Because, as you may have guessed, the Middle East was not always a vast desert. Around 100 million years ago the region was covered by what scientists call the Tethys Ocean. Rivers feeding this ancient ocean saturated it with nutrients, giving rise to massive numbers of microscopic animals destined to be pressure cooked into oil. The Tethys eventually receded and gave way to the sandy Middle East we know today. But it left behind a reminder of the area’s watery past in the form of vast oceans of oil buried beneath the sand.