Bears Ears was not the only monument Obama made, nor was it the largest. But more than six months after its creation, it is still the most polarizing. When President Donald Trump ordered the Department of Interior in April to consider if any national monuments created since 1996 should be rescinded, many assumed that he would target Bears Ears for special consideration. Trump also called Obama’s creation of many national monuments an “egregious abuse of executive power.”

“It’s gotten worse and worse and worse, and now we’re going to free it up,” Trump added.

This week, Ryan Zinke, the secretary of the interior, delivered a preliminary report to President Trump recommending that Bears Ears be reduced in size. In the leaked copy of the memo, first published by Deseret News, Zinke does not provide specifics about how or where the monument should shrink.

It’s a little unclear, however, whether Trump can do that.

The United States is very big, and a little less than a third of it is owned by the government. The Constitution gives Congress the sole responsibility to manage and dispose of all that land, but it can delegate some of that power to another branch by passing a law.

In 1906, Congress passed the Antiquities Act, empowering the president with the ability to create national monuments—a kind of second-tier national park—when federal land contains objects that are threatened by outside forces or which are especially deserving of emergency protection. The act was explicitly passed to shield sites of historical or indigenous importance from “pot hunting,” in which Americans would loot artifacts from archeological sites or abandoned dwellings and then sell them on the illicit market.

Since then, presidents have used the law to create a new reserve 151 times. They have protected small sites and big ones. They have made many national monuments that then, a decade or two later, became a national park—including Grand Canyon, Bryce, Zion, Acadia, Olympic, and Grand Teton. (Many times, a president locked up land in a national monument after becoming frustrated with Congress’s failure to make it into a national park first.)

A president has never undone a national monument after its creation, however. That’s what Utah senators Mike Lee and Orrin Hatch have demanded Trump do to Bears Ears: completely reverse Obama’s decision to designate it as a national monument.

So far, Zinke’s proposal seems slightly less dramatic. He recommends that the “boundary [of Bears Ears] be revised through the use of appropriate authority, including lawful exercise of the president’s authority” under the Antiquities Act.

“Rather than designating an area encompassing almost 1.5 million acres as a national monument, it would have been more appropriate to identify and separate the areas that have significant objects to be protected,” Zinke says in the memo. “Additionally, many of the lands in the [monument] are already congressionally or administratively protected.”