Its vast American empire included 20,000 domestic wells, 4,000 miles of pipeline, 5,000 tank cars, and over 100,000 employees. Its share of world oil refining topped out above 90% but slowly dropped to about 80% for the rest of the century. In spite of the formation of the trust and its perceived immunity from all competition, by the 1880s Standard Oil had passed its peak of power over the world oil market. Flagler and Rockefeller finally gave up their dream of controlling all the world's oil refining. Rockefeller admitted later, "We realized that public sentiment would be against us if we actually refined all the oil." Over time foreign competition and new finds abroad eroded his dominance. In the early 1880s, Flagler and Rockefeller created one of their most important innovations. Rather than try to influence the price of crude oil directly, Standard Oil had been exercising indirect control by altering oil storage charges to suit market conditions. Flagler and Rockefeller then decided to issue certificates against oil stored in Standard Oil's pipelines. These certificates became traded by speculators, thus creating the first oil-futures market which effectively set spot market prices from then on. The National Petroleum Exchange opened in Manhattan in late 1882 to facilitate the oil futures trading.