National Petroleum Reserve, area, c.23 million acres (9.32 million hectares), Alaska North Slope, situated W of Prudhoe Bay and the Arctic National Wildlife Refuge. The reserve, which is the largest remaining parcel of unprotected public land in the United States, overlies a petroleum deposit estimated at 6–13 billion barrels. Designated the Naval Petroleum Reserve Number 4 in 1923 and run by the U.S. Navy, it passed to the federal Bureau of Land Management (BLM) in 1976, when its present name was adopted. From the mid-1990s to the mid-2000s the BLM sold oil leases for the NW and NE areas of the reserve. A plan for additional oil and gas drilling in the SW reserve was formulated in 1998 and later expanded (2004) to include opening most of a 9.2-million-acre (3.7-million-hectare) section of the area to drilling. Several environmental groups have objected to the plans, citing dangers to wetlands and wildlife and threats to the livelihood of indigenous people.