In-situ combustion is a method used to extract oil from underground reservoirs. This technique involves igniting a portion of the oil in place, which generates heat and helps to reduce the viscosity of the remaining oil, making it easier to flow to production wells. The process typically starts with the injection of air or oxygen into the reservoir, which supports combustion. As the fire burns, it creates a combustion front that moves through the oil, enhancing recovery rates. This method is often applied in heavy oil fields, where traditional extraction methods may be less effective.