The Trump administration proposed on Thursday to open more than two-thirds of the nation’s largest piece of public land to oil and gas drilling, removing wildlife protections for the Alaskan tract that have been in place for more than four decades. The Interior Department’s Bureau of Land Management wants to allow fossil fuel extraction in roughly 82 percent of the National Petroleum Reserve-Alaska on the state’s North Slope. Less famous than the nearby Arctic National Wildlife Refuge, it is one of the most ecologically valuable tracts of federal property — providing a critical refuge to polar bears as well as tens of thousands of migrating caribou and waterfowl.

The reserve, about the size of Indiana, is also one of the most promising onshore oil prospects in the country. A recent analysis by the U.S. Geological Survey offered a mean estimate of 8.7 billion barrels in undiscovered oil and 25 trillion cubic feet of natural gas.

The BLM posted notice of its Final Environmental Impact Statement on Thursday, and is expected to issue a final Record of Decision within 30 days. Environmentalists and some Alaska Natives, who have lived on the North Slope for millennia and depend on its game for subsistence, are likely to challenge the decision once it’s final.