Extensive environmental protections put in place by former President Barack Obama in waters surrounding the Northwestern Hawaiian Islands could be in jeopardy following an executive order issued by President Donald Trump on Wednesday that requires a review of national monuments.

Several months prior to leaving office, Obama expanded the Papahanaumokuakea Marine National Monument by hundreds of thousands of square miles, creating the world’s largest marine reserve and protecting coral reefs and deep sea marine habitats from activities such as commercial fishing and mineral mining.

The designation quadrupled the size of the protected area surrounding the Northwestern Hawaiian Islands, which was first designated as a national monument by President George W. Bush in 2006.

The area is one of about two dozen national monuments created by presidents in the past two decades under the Antiquities Act of 1906 that Trump has instructed the Interior Department to review.

Trump described the move as part of his larger effort to remove “job-destroying regulations” and return power to the people during a news conference on Wednesday.

“The Antiquities Act does not give the federal government unlimited power to lock up millions of acres of land and water, and it is time we review this abusive practice,” said Trump, calling the designations a “massive federal land grab.”

“It’s gotten worse and worse and worse and now we are going to free it up,” Trump said. “This should never have happened.”

Most national monuments, including Papahanaumokuakea, were already on lands controlled by the federal government.

The Antiquities Act, which affords presidents unilateral power to designate U.S. lands and waters as national monuments, was signed into law by President Theodore Roosevelt in response to years of looting and destruction of Native American sites in the Southwest. The law was put in place as a way to ensure the protection of the country’s natural resources and historic sites, and includes such areas as Grand Canyon National Park.

However, the practice of designating lands as national monuments has been controversial among conservative groups, such as the Heritage Foundation, a right-wing think tank that has called for repealing the law. Some designations also have stirred opposition within local communities, including in Hawaii.

The effort last year to expand the Papahanaumokuakea Marine National Monument was opposed by the local leaders of the longline fishing industry, which primarily employs foreign workers under conditions that have recently stirred controversy. Former U.S. Sen. Daniel Akaka and former Hawaii Gov. George Ariyoshi also registered their opposition to the designation.

Supporting the expansion were a number of prominent Native Hawaiian leaders, U.S. Sen. Brian Schatz and Hawaii Gov. David Ige, among others. State lawmakers were split over the issue.

Concerns quickly surfaced among environmental groups after Trump’s election that the president would try to roll back the Papahanaumokuakea Marine National Monument. Trump had vowed to reverse Obama executive orders and reduce environmental regulations.

Those fears mounted last month when environmental groups learned that the Western Pacific Regional Fishery Management Council, a quasi-governmental agency charged with helping manage Pacific fisheries, sent a letter to Trump asking him to reverse the designation.

Paul Achitoff, an attorney with Honolulu’s Earthjustice who has supported the monument designation, said Trump’s action on Wednesday was not unexpected, but “still sad.”

“We have been expecting something along these lines for a while,” Achitoff said. “It is premature to be saying what exactly will be coming out of it, but if it turns out to be an attempt to roll back the monument, we will be fighting it and we think we will win.”

There is legal debate over whether a president actually has the power to revoke a national monument. While presidents can modify a monument designation, the 1906 act is silent on whether a president can revoke it entirely, according to research published in 2000 by the Congressional Research Service. Revoking a designation could require approval from Congress.

Achitoff said he doubts Trump would be able to successfully revoke a monument designation.

“He can try. He has been trying all sorts of things that haven’t worked and this would be another one,” he said.

Schatz, whom Obama singled out last year as being instrumental in securing the expansion of the monument area, issued a news release saying that Trump’s executive order on national monuments will “prove to be a waste of federal resources.”

Schatz said that the expansion had widespread support from the public and from Hawaii’s political leaders and that the area’s ban on fishing hasn’t hurt the profits of the longline industry.

“The longline industry has suffered no economic impact,” he said in the news release. “It has just had its most profitable year, just after expansion occurred, and this year will likely prove to be even more profitable.”