Nigeria is the largest producer of crude oil in Africa and the fifth-largest supplier of crude oil to the United States. (One of its blends, called "Bonny light" after a town and oil port in Rivers state, is highly valued because of the ease with which it can be refined into high-quality gasoline.) Nigeria's oil output, however, has been reduced 20 percent over the last two years by the activities of an indigenous militant group that the world knows very little about. In 2006 and 2007, attacks on oil infrastructure in the Niger Delta by the Movement for the Emancipation of the Niger Delta (MEND) contributed to a spike in global oil prices — and to Nigeria’s placement high on the list of countries critical to U.S. energy security. For all its influence on oil prices, MEND has played its cards close to its chest.