Although fossil fuels like coal have been harvested in one way or another for centuries, crude oil was first discovered and developed during the Industrial Revolution, and it's industrial uses were first developed in the 19th century. Newly invented machines revolutionized the way we do work, and they depended on these resources to run. Today, the world's economy is largely dependent on fossil fuels such as crude oil, and the demand for these resources often spark political unrest, since a small number of countries control the largest reservoirs. Like any industry, supply and demand heavily affects the prices and profitability of crude oil. The United States, Saudi Arabia, and Russia are the leading producers of oil in the world.