OPPORTUNITY, Mont. — For a century, this tiny town has breathed beneath the long shadow of a colossal chimney. In fact, a mining behemoth created Opportunity specifically to prove it was safe to live in the path of the dark plume of smoke that billowed from the company’s copper smelter.

The plant is long shuttered, but the big stack looms 585 feet tall, the largest free-standing masonry structure in the world. Studies long ago shattered the company’s safety claims by linking high levels of arsenic and lead to serious health threats. These days, Opportunity is defined by its struggle to force powerful corporate and government interests to clean up the dangerous waste left behind.

After more than a decade of court battles, the Supreme Court on Tuesday will hear residents’ case against the property’s current owner. Theirs is more than just a David vs. Goliath confrontation — a rural community of 700 against Arco, a subsidiary of the oil giant BP — because of its potential for upending one of the nation’s key environmental laws.

Opportunity and its even smaller neighbor, Crackerville, want the justices to uphold a Montana Supreme Court ruling that found federal Superfund law does not override this state’s constitutionally guaranteed right to a “clean and healthful environment.” Arco, which appealed the decision, says it has already spent $470 million to reduce arsenic levels in the area. The towns say an additional $58 million is needed to lower those levels far more drastically.

The outcome of Atlantic Richfield Co. v. Christian holds significance well beyond this scenic valley in southwest Montana. Arco, as it is now known, argues that allowing individuals to sue in state court for greater cleanup than federal regulators require would nullify a key component of the Superfund program.

“The decision threatens the integrity of every future [Superfund] settlement EPA enters,” Arco has warned. The U.S. Chamber of Commerce and several industry groups agree, predicting “chaos” for sites nationwide should Opportunity prevail.

“It’s a really long struggle,” said Serge Myers, who in the early 2000s helped spearhead a citizens’ movement that led to the lawsuit. He is now a 74-year-old great-grandfather. “We only have one lifetime, and the corporations have forever. We just want our yard to be clean and healthy for our kids.”

For Myers, the dormant smelter stack represents the best job he ever had. He worked nearly 17 years at the Anaconda Copper Mining Co. plant, which was the lifeblood of the city of Anaconda and outlying communities. He started there in the early 1960s and moved through a series of jobs until the company sold its properties to Arco in 1977.

“The money was good, we were all union members. By the time it ended, I had five weeks of vacation a year and good benefits,” Myers recalled. “We had good jobs. We knew it was under smoky skies, but it was just part of the life, I guess. In smeltering cities, you just deal with it.”

Though the plant’s operations had slowed, Arco still stunned the region when it shut down the entire operation in November 1980 and eliminated the 1,000 remaining jobs. Even so, the people of Opportunity couldn’t have foreseen that the legal fight over toxic waste would outlive many of them. Myers guesses that a dozen of the nearly 100 residents who brought the lawsuit have died.

“I certainly thought we’d be treated better by the company than what we were,” he said.

Opportunity and Crackerville are part of the country’s biggest Superfund complex, toxic relics from the century of copper mining. The ore lay under nearby Butte and was extracted through a massive maze of tunnels and later an open pit; Anaconda, 30 miles away with plentiful water, got the giant smelter that processed the ore.

In the aftermath of the closure, as the region reeled economically and socially, residents began to piece together how much environmental damage the mega-mining had done.

Things got even worse around 2005, when bigger and much wealthier Missoula, 110 miles away, jumped to the front of the cleanup line as a dam with dangerous levels of toxic sludge was removed. Some 2.2 million cubic yards of waste were shipped back up the Clark Fork River and heaped on an already problematic dump site just outside Opportunity.

Robin Saha, an environmental studies professor at the University of Montana, began working with residents as troubles started to pile up. Cancer-causing arsenic and other heavy metals had infiltrated the land and nearby streams, and dust storms — their contents unknown — plagued the town.

Myers had collected reams of evidence on the waste but had no experience in getting the attention of environmental groups or government. Saha and a team of graduate students helped Opportunity organize.

“Usually we think of environmental justice involving communities of color being dumped on,” he said. “Environmental justice is also about low-income and communities that lack the know-how and resources to stand up for themselves. That description fit Opportunity to a T.”

The core issue now in court is arsenic. In the early years, residents shared stories of mass sickness and death among livestock. In more recent times, they’ve worried over land where grass just wouldn’t grow. Shaun Hoolahan, a petroleum engineer who grew up here but has lived all over the world, points to the benign tumors on his dog’s belly as he describes how she used to lie on a bare spot of his yard.

“That’s got me wondering, what’s really safe,” he said.

In their suit, residents are asking Arco to reduce arsenic levels in soil throughout the community to pre-contamination levels, around 10 times lower than the EPA-approved cleanup that brought them down to 250 parts per million. The company has pushed back against further remediation, saying “respondents’ soil-excavation plan risks spreading currently contained arsenic everywhere.”

If the state court ruling is upheld, Arco contends, “At any time, any landowner on any Superfund site could decide to shred EPA’s cleanup plans and impose different, and potentially detrimental, multimillion-dollar cleanups. The entire community would be hostage to the whims of a small minority of dissatisfied landowners.”

Hoolahan, who moved home to care for elderly parents, worries about both the safety of the town and the stigma attached to living in a Superfund site. Opportunity lies in the shadow of the stunning Pintlar mountain range, within easy distance to pristine hiking trails, lakes and skiing, but it’s never become a recreational hub or a place with strong property values. Hoolahan rubs his forefingers and thumb together — the symbol of money — as he talks about the reason.

“If this were Palm Springs, it would be a whole different matter,” he said.

People here are wary of the Supreme Court’s involvement, in part because the attorney arguing Arco’s case was a prominent supporter of Justice Brett M. Kavanaugh’s nomination last year. While such connections are not unusual in Washington, it does add to a decades-long list of actions that have made Opportunity residents feel like the deck is stacked against them.

No matter what the justices conclude, one of the residents’ lawyers stresses that Opportunity’s effort won’t end. According to Justin Stalpes, there are other legal avenues yet to pursue.

“The people of Opportunity,” Stalpes noted, “have been fighting the powers that be since the inception of the town.”