American refineries have worked steadily over the years to improve methods of operation aimed at increasing efficiency with startling results. Although the number of U.S. refineries dropped 50 percent in the past 25 years due to closures, refinery output has increased over 25 percent, as remaining refineries have expanded through de-bottlenecking, higher utilization and technology advancements. Over the past decade ExxonMobil alone has added the equivalent of three new refineries simply by expanding already-existing facilities at a fraction of the cost of constructing new ones.