(CN) — The Trump administration opened the largest remaining area of pristine land in the country to oil drilling Monday, approving a highly controversial oil and gas leasing program within Alaska’s Arctic National Wildlife Refuge.

Alaska Governor Mike Dunleavy estimated there are between 4.3 and 11.8 billion barrels of recoverable oil reserves below the refuge.

“Today’s announcement marks a milestone in Alaska’s 40-year journey to responsibly develop our state and our nation’s new energy frontier,” Dunleavy said in a statement.

The program was originally built into a 2017 tax act by Alaskan Senator Lisa Murkowski and supported by other Republicans who championed the move as a revenue generator for the federal treasury. Congress approved the program in 2017, and the Interior’s Bureau of Land Management in December 2018 concluded drilling could be conducted within the coastal plain area without harming wildlife.

After signing the record of decision Monday to lease the 19-million-acre area, Interior Secretary David Bernhardt called the move “a significant step.”

But opponents of the leases say development poses a threat to the polar bears and Porcupine caribou populations who live there, as well as members of the native Alaskan Gwich’in Tribe whose food, cultural and spiritual needs rely on the migration and hunting of caribou. Their community is located a few hundred miles from the refuge.

“There’s no good time to open up America’s largest wildlife refuge to drilling, but it’s absolutely bonkers to endanger this beautiful place during a worldwide oil glut,” Kristen Monsell, an attorney in the Center for Biological Diversity’s oceans program, said in a statement. “An oil spill in this special sanctuary could devastate polar bears and caribou and cause irreparable harm to a pristine Arctic ecosystem. We’ve reached a dangerous new low in the Trump administration’s obsession with expanding the extraction of dirty fossil fuels.”

The U.S. Fish and Wildlife Service has testified to the area’s significant ecological diversity as well.

“Caribou and moose roam freely throughout the refuge,” the agency said in a 2019 report. “Black, brown, and polar bears all make their home here. Arctic fox, wolves, and wolverine hunt for ptarmigan, snowshoe hares and arctic ground squirrels. Located at the northern end of all four North American flyways, the refuge hosts more than 200 species of birds through the year. Arctic grayling, arctic char and Dolly Varden are three of the more than 40 fish species in the fresh and marine waters of the refuge.”

Though House Democrats passed a bill last year to protect the area from developers in a 225-193 vote, the measure failed in the Republican-controlled Senate in a 52-48 vote.

Once oil companies gain land rights to the undeveloped land, it will be substantially harder for Democrats to stop development even if they take the White House or Senate and keep the House in the November election. At a campaign event in early 2020, Democratic nominee Joe Biden said that he’s “totally opposed” to drilling in the refuge.

Several environmental groups say they plan to file a legal challenge soon.

In a statement Monday, Adam Kolton, the executive director of the Alaska Wilderness League Action, noted that the Trump administration’s move comes at a time when oil prices have plummeted and major banks are pulling out of Artic financing. The Trump administration’s liquidation of the nation’s wilderness will not go unnoticed, he said.

“We will continue to fight this at every turn, in the courts, in Congress and in the corporate boardrooms,” Kolton said. “Any oil company that would seek to drill in the Arctic Refuge will face enormous reputational, legal and financial risks.”

Gina McCarthy, president of the Natural Resources Defense Council echoed this sentiment, saying that her organization “will never stop suing them” for pushing drilling in these lands.

“This is an egregious intrusion into the sacred lands of the Gwich’in and other Indigenous People. It threatens the heart of the largest pristine wildland left in America — the birthing grounds and nursery for the Porcupine Caribou Herd, and home to polar bears, musk oxen, migratory birds and other precious wildlife,” McCarthy said. “The administration’s reckless, relentless boosting of the oil industry will irrevocably damage this cherished place and compound the global climate crisis.”

According to a report by the NRDC, drilling in this area would require “a vast network of roads and pipelines that would disastrously fragment habitat and displace wildlife,” to say nothing of the danger of oil spills in the region.

The area has been a battleground between conservationists and oil developers since 1980 when Congress earmarked refuge for possible oil drilling ventures. While Republicans have attempted to push drilling in the area for the last four decades, Democrats have held them off up until now. President Bill Clinton vetoed a drilling bill in 1995. Democratic senators filibustered a similar bill in 2005.

During a 2018 speech at a GOP conference, Trump said that he “really didn’t care” about opening the refuge for drilling until he learned about the repeated attempts and failures of previous Republican presidents since Ronald Reagan. Afterward, he pushed for it to be included in the 2017 tax act.

“I really didn’t care about it, and then when I heard that everybody wanted it — for 40 years, they’ve been trying to get it approved, and I said, ‘Make sure you don’t lose ANWR,’” Trump said.