The Supreme Court on Monday heard arguments in a high-profile case that could block construction of an $8 billion gas pipeline seeking to cross the Appalachian Trail.

The proposed Atlantic Coast Pipeline (ACP) would carry natural gas 604 miles from West Virginia to North Carolina and would tunnel below the famed trail that runs more than 2,000 miles from Georgia to Maine.

At issue is whether jurisdiction over the affected land belongs to the U.S. Forest Service or the National Park Service (NPS). The case presents the justices with a complex tangle of federal laws that will determine if the land is open to energy development or must be preserved for recreational use under the park service's mandate.

The Forest Service issued a permit that would allow the pipeline to cross the trail, but that decision was challenged in the courts by a number of environmental groups. They argue that because the National Park Service oversees all of the federal lands that make up the trail, other agencies don't have authority to issue a permit. And because the park service's mandate is focused on conservation, only an act of Congress could allow the pipeline to cross the trail.

The 4th Circuit Court of Appeals agreed, revoking the Forest Service permit.

The government and the Atlantic Coast Pipeline - on the same side in this case - appealed the decision to the high court. The federal government's lawyer on Monday argued that the Forest Service had the right to issue a permit.

"If a tree falls on Forest lands over the trail, it's the Forest Service that's responsible for it. You don't call the nine Park Service employees at Harpers Ferry and ask them to come out and fix the tree," argued Anthony Yang with the Office of the Solicitor General, referring to one of the main park service outposts along the trail.

In the lower court, lawyers for the Southern Environmental Law Center (SELC) argued the permit to cross the Appalachian Trail was only secured due to a change in administration, the first in the trail's roughly 50-year history. That permit, they argued, threatened pipeline construction over steep and sensitive terrain before tunneling 600 feet below the trail, creating a "scar" on the landscape.

"ACP developers should be playing by the rules, but instead they used political pressure to push a risky project through that, in the end, would harm our public lands," SELC lawyer DJ Gerken said in a press conference after the hearing.

The Atlantic Coast Pipeline is already tied up in a number of other legal challenges and lacks several other required permits, but the Supreme Court case could determine the fate not only of this pipeline, but others that seek to cross Appalachia.

In a 2018 decision, the U.S. Court of Appeals for the 4th Circuit sided with the environmental activists based on its interpretations of several interconnected federal laws.

The court determined that a 1968 statute called the National Trails Act transferred control over the Appalachian Trail to the National Park Service. And a separate law, the Mineral Leasing Act, prevents the NPS from granting land access, known as a right-of-way, for energy development.

But lawyers advocating for the pipeline construction say the lower court misread the law. They argue the National Trails Act did not wrest control of the Appalachian Trail from the Forest Service, which, unlike the NPS, does have the power to grant rights-of-way.

Justices expressed concerns about the implications of drawing lines between the overlapping authority of two agencies on state and city rights, as well as the future of American energy development.

They questioned if siding with environmental groups could lead the Appalachian Trail to become a sort of Great Wall, blocking pipelines moving from West to East.

Chief Justice John Roberts asked whether siding with environmentalists would lead to a sort of "impermeable barrier" blocking resources in the West from areas to the East with need for them.

"Absolutely incorrect," Michael Kellog, who represented the environmental groups before the Supreme Court, told Roberts.

Kellog argued that wasn't a concern because pipelines and other infrastructure could continue to cross through the private and state lands that already are home to roughly 35 pipelines. Another 19 were approved on federal lands before they were designated as part of the trail.

"In the past 50 years, since the Trails Act was passed, there has not been a single new pipeline right-of-way through federal lands under the Appalachian Trail. There have been a number on state, local, and private lands," he added.

Atlantic Coast Pipeline lawyer Paul Clement told the justices that affirming the lower court ruling that the trail was under the purview of the park service could have far reaching effects on large swaths of private, state and even urban land.

"The Park Service administers the Selma to Montgomery National Historical Trail. Now, if that trail is 'lands' in the Park Service because we just can't get our head around the idea that trails are different from land, then parts of downtown Selma, downtown Montgomery are lands of the National Park Service."

Several justices expressed concerns over potentially expanding the NPS's authority in this way - under what liberal Justice Sonia Sotomayor referred to as "the nightmare" scenario the pipeline advocates described.

"It runs through cities, the trail. It runs through downtown Selma," she said. "Does the Park Service, can it regulate the use of that area? Can it put, at a halt, everything that happens in downtown Selma?"

Beyond the legal issues, the case raised a number of semantic questions: Is a trail simply a footpath running along the earth's surface? Or does it encompass the lands through which it traverses?

This question bothered Justice Elena Kagan, who said the government and pipeline lawyers were engaging in verbal gymnastics to try to separate a trail from the land on which it sits.

"You're great brief writers and you're great writers, and the briefs are strange to read because you can't ever just say what you mean," Kagan said. "Which is that the trail is a piece of land, so you find yourself wrapped up in these strange locutions about the trail traversing land. It's like you're imagining something that goes on top of it somehow."

That issue persisted throughout the oral arguments, with justices posing various questions on the depth of the land overseen by the government and how it is administered by federal agencies.

"I don't really think it's as metaphysical as you think. I mean, the philosophers at the Park Service and the Forest Service haven't had any problem with this for 50 years," Clement said to the justices.

The SELC has already won another case against the pipeline. In January, the 4th Circuit Court of Appeals rejected another permit needed for the pipeline to pass through Union Hill, Va., a predominantly African American community founded by freed slaves after the Civil War.

Update at 1:46 p.m.