President Trump signed an executive order Wednesday instructing the Interior Department to review the national monuments created by his three most recent predecessors. Trump appears to be signaling his willingness to revoke national monument status from some of these sites, particularly Bears Ears National Monument in Utah. Former President Barack Obama designated the Bears Ears site, which covers nearly 1.4 million acres, in the final month of his administration. The creation of such a large monument raised the ire of conservatives including Utah Sen. Orrin Hatch, who called it federal government overreach.

Presidents can designate national monuments without congressional approval thanks to the Antiquities Act, signed by President Teddy Roosevelt in 1906. National monuments receive protection on par with national parks, preventing activities that might alter or damage the site. But since that act, no president has ever revoked a site’s status, according to data collected by the National Parks Conservation Association, a nonprofit group that promotes the conservation of public land. National monument status has been revoked from only 11 sites (many others have been redesignated as national parks or other kinds of nationally protected sites). And in each of those cases, Congress, not the president, took away the site’s national monument status. In fact, the Antiquities Act does not address whether presidents even have the power to rescind national monument status, and legal scholars disagree on this point. If Trump removes Bears Ears’ national monument designation, he’ll be testing new ground.

National monuments make up only a small share of all the land owned by the federal government. In the 50 states, national monuments designated under the Antiquities Act cover about 12 million acres of land. However, about half of these national monuments, including the four largest and several others over the 100,000-acre threshold, were all established within the past three administrations and are therefore subject to review under Trump’s executive order.

Overall, the U.S. government owns about 640 million acres, mostly in the West. This land is managed by several federal agencies (the responsibility for national monuments falls to different agencies, depending on the monument):

That doesn’t mean the western half of the country is just one giant park kept in its pristine, natural state. The U.S. Geological Survey maintains a database that aggregates data from national, regional, state and local agencies to try to map all the public land in the U.S. It also evaluates whether that land has a designation (such as national monument) that offers it some kind of protection. Much of the land owned by the government does not have any special designation. In those areas the government has much freer reign in determining what will be allowed on the land, including activities such as cattle grazing, drilling and logging.

So, even before this executive order, Trump had the power to reshape how the nation was using its public lands. And he used it: Since Trump took office, his administration has repealed a ban on lead fishing lures and bullets on federally owned lands and issued an executive order to review a rule that extends federal protection to smaller bodies of water. Trump has also signed a joint resolution making it easier for coal companies to operate on public land. This latest order is only the most recent, though perhaps the most symbolic, indication that the Trump administration is pointedly rejecting Obama-era conservation policies.

That might not go down smoothly with the American people. National parks have never been more popular, and in a NBC News/SurveyMonkey poll conducted in February, 47 percent of respondents said more U.S. land should be protected by national parks, while 40 percent said the amount currently protected was about right. Only 9 percent said less land should be protected. In the West, home to most of this land, the popularity of recreational activities such as hunting and fishing means that many conservatives support conservation. In fact, one of those conservatives is Trump’s secretary of the Interior, Ryan Zinke, who is from Montana and describes himself as a “Teddy Roosevelt conservationist.” But while Teddy Roosevelt established the first national monument, Zinke’s boss may be the first president to abolish one.