President Donald Trump’s executive order seeking to find new ocean expanses in the Atlantic and the Arctic for offshore drilling is unlikely to reach its goals anytime soon, but instead will kick off a yearslong review and a legal battle.

The day before his 100th in office, Trump took his step Friday toward dismantling a key part of Barack Obama’s environmental legacy.

“This executive order starts the process of opening offshore areas to job-creating energy exploration,” Trump said at a White House ceremony. “It reverses the previous administration’s Arctic leasing ban and directs Interior Secretary Ryan Zinke to allow responsible development of off-shore areas that will bring revenue to our treasury and jobs to our workers.”

Despite Trump’s assertion that the nation needs to wean itself of foreign oil, U.S. oil imports have declined in recent years as domestic production boomed, mainly through improved drilling techniques that opened up production in areas once out of reach.

And environmental law and policy experts questioned Trump’s authority to reverse Obama’s withdrawal of certain areas in the Arctic or Atlantic to drilling, a question that will likely be decided in the courts.

“It’s not quite as simple as the president signs something and it undoes the past,” said Sean Hecht, a University of California, Los Angeles environmental law professor.

For instance, Obama had used his authority under the Outer Continental Shelf Lands Act to protect Arctic areas from oil drilling late last year, a move Trump’s order seeks to undo. At the time, Obama administration lawyers said they were confident that move would be upheld in court.

Legal experts say the law has never been used by a president to remove protections, just to create them.

“The statute doesn’t allow that. It allows the president to put land within a protected zone but says nothing about allowing president to take it out,” said Rob Verchick, an environmental law professor at Loyola University in New Orleans.

Verchick, a policy administrator at the U.S. Environmental Protection Agency under Obama, added: “I suspect it will be fought in the courts.”

Trump’s order also directed Commerce Secretary Wilbur Ross to conduct a review of marine monuments and sanctuaries designated over the past 10 years. Obama issued monument proclamations under the Antiquities Act, including the Northeast Canyons and Seamounts Marine National Monument in the Atlantic, which protected that swath of sea from drilling.

Legal scholars said Trump would enter uncharted territory if he seeks to undo a national monument proclamation in an effort to remove environmental protections. The president could issue a new proclamation eliminating a specific monument, but since it’s never been done, the courts again would likely decide whether the tactic is allowed under the act.

“I believe that a president does not have the legal authority to do it,” said UCLA’s Hecht.