Bethpage, Long Island, was an American dream come true for the Cornett family.

“Baseball fields, swimming pools, great schools, best community,” Bruce Cornett, 67, rattled off.

“Who would have ever thought that there was this monster underneath you in the ground?”

The “monster” is a six-square-mile toxic plume beneath the town that has been creeping into the air, water supply and even homes for decades — and is still spreading toward the Great South Bay.

The Cornetts believe the monster struck three of them down with cancer within 20 months of each other — a remarkable cluster, even in Bethpage, where residents have long argued cancer is more prevalent than surrounding areas.

The family has filed a $300 million federal lawsuit against aviation giant Northrop Grumman for its years of hazardous waste dumping, which led to the underground contamination.

“Whoever is culpable, they’re going to have their time in court,” said Bruce’ son, Christopher, 34. “This shouldn’t have happened.”

In December 2015, mom Cathy, 66, was diagnosed with kidney cancer. Six months later, Christopher was given a 5 percent chance to live after doctors discovered aggressive testicular cancer. And in August 2017, Bruce was treated for prostate cancer.

They were all healthy before their shocking diagnoses. Only daughter Lauren, 32, does not have cancer.

Northrop Grumman knowingly infested Bethpage’s air, soil and water with carcinogens for decades, the family charges in the Brooklyn Federal Court claim.

“I think we paid our fair share, having 75 percent of our household get cancer,” said Christopher, a lifelong athlete who survived intensive chemotherapy and three surgeries as doctor’s fought to control his illness, which was 10 times more aggressive than Lance Armstrong’s testicular cancer.

The aviation company, once Long Island’s biggest employer, built famed Navy World War II aircraft like the F4F Wildcat at their sweeping Bethpage campus.

But it also dumped carcinogens on land which later became the community park — where kids like Christopher spent years playing Little League and attending summer camp. Last year, officials found radon gas in Bethpage schools.

Authorities say Bethpage’s water is safe. But residents scoffed at a 2013 state Department of Environmental Conservation study which found Bethpage cancer rates were similar to surrounding areas.

Officials have monitored and treated local water and public drinking wells since the 1970s, said the DEC, which said the state expanded efforts, and is using new technologies, to battle “the plume.”

But as new carcinogens are identified, it casts doubt on past water remediation at the longtime Superfund site, said lawyer Nicholas Rigano, who reps the Cornetts.

“The treatment may not have been fully effective,” he said, noting past remediation of wells wasn’t capturing then-unidentified contaminants such as radium, 1,4-dioxane, and others.

Northrup declined comment on the lawsuit, adding it’s worked with authorities to address Bethpage’s environmental issues for 20 years.

But Christopher Cornett says the pace is too slow for people like him and his parents – who now live in fear their other daughter will be diagnosed.

“I just hope to God she’s spared,” said Bruce Cornett, who recalled sitting at his wife’s first birthday dinner after her surgery to remove her cancerous kidney as Christopher complained of pain in his sides.

The next day, Christopher checked into the hospital and was shocked to learn of his diagnosis.

At first they kept the new from his mom.

“I guess in my own my mind I was trying to make it that it wasn’t as severe as it was, that he would be able to get through, that he’s in the hospital, he’s going to come back out,” said the dad, choking back tears.

“And I did, Dad. I did,” Christopher said, comforting him.

“I don’t think people understand how big this problem is,” he said.