On January 11, 2006, the U.S. Department of the Interior (DOI) approved oil and gas drilling on approximately 500,000 acres (2,000 km2) of land in and around Teshekpuk Lake on Alaska's North Slope within the National Petroleum Reserve-Alaska. Up to 90,000 geese congregate in this area in summer to undergo wing molt, and up to 46,000 caribou use the area for both calving and migration. Some environmental groups contested the DOI decision to allow drilling. The decision stipulated that no surface drilling would be allowed on land considered crucial for molting geese or caribou, and a maximum of 2,100 acres (8.5 km2) in seven different zones could be permanently disturbed on the surface.