In the ground under ANWR is one of the last major oil reserves in the United States. There are debates on whether or not to allow oil drilling. Those in favor have argued that removing the oil will make the U.S. less dependent on foreign supplies of oil. In the 1990s, the U.S. imported over half of the oil it used. ANWR is also near the Prudhoe Bay Oil Field. It contains large oil deposits that are already providing oil. But, the refuge land is a fragile natural environment. The area proposed for oil drilling is also the calving grounds for the Porcupine caribou herds.