Say the phrase “U.S. nuclear accident” and those of us old enough might think of Three Mile Island, a nuclear plant in Pennsylvania shut down in March 1979 after a series of errors, both mechanical and human.

The accident generated scary headlines and later came to symbolize the nation’s conflicted relationship with nuclear energy.

But the largest spill of radioactive material in the United States happened three and half months after Three Mile Island and several thousands of miles west of Pennsylvania.

July 16 will mark the 35th anniversary of the day in 1979 when a dam on the Navajo Nation near Church Rock, N.M., broke at an evaporation pond, releasing “94 million gallons of radioactive waste to the Puerco River, which flowed through nearby communities,” according to a May 2014 report from the U.S. General Accountability Office.

The radioactive material was a mixture of water and mill tailings, leftovers that retained toxic contaminants from the mining process that converted mined uranium into yellow slurry, known as yellow cake. The tailings were “placed in unlined evaporation ponds at the mill site,” the report says, meaning the radioactive goop that washed into the Puerco River and flowed through communities downstream was a public health hazard.

Perhaps for many New Mexicans, the 1979 dam release might be old news. But growing up in another region of the country, it’s still pretty fresh for me. I didn’t know about it until recently and when I first learned of it I wondered why I hadn’t heard about in history books or news coverage. Maybe I missed it.

The dam collapse appears to have been a singular event on the reservation, which is about the size of West Virginia, more than 25,000 square miles.

Former Los Angeles Times reporter Judy Pasternak has explored the legacy of uranium mining on the Navajo reservation in her 2010 book, Yellow Dirt.

Uranium mining on the Navajo reservation started in the 1940s, when the United States was stockpiling nuclear weapons, and lasted until the mid 1980s. During that period, federal officials estimate that private companies mined approximately 4 million tons of uranium ore from mines on the Navajo reservation.

The U.S. Department of Energy has estimated that “millions of gallons of water contaminated by mill tailings were released into the groundwater over the life of the sites through the unlined ponds,” according to the GAO report.

But the 1979 dam collapse represented something altogether different than the leaching of contaminated materials into the groundwater, which occurs over time. The onrush of contaminated water was a fast-moving public health disaster, with almost immediate effects.

Pasternak, who dug into archival collections, books and government documents and who interviewed hundreds of people for Yellow Dirt, described the consequences of having contaminated water come into contact with humans and animals on July 16 and the days afterward:

The water, filled with acids from the milling process, twisted a metal culvert in the Puerco and burned the feet of a little boy who went wading. Sheep keeled over and died, and crops curdled along the banks. The surge of radiation was detected as far away as Sanders, Arizona, fifty miles downstream. The IHS (Indian Health Service) and the state urged Navajos not to drink the water nor enter it, nor let their animals do so, anywhere downstream from the spill. But the people by the Puerco didn’t have many alternatives.

The dam collapse was one of the more public catastrophes due to decades of uranium mining on the Navajo Nation. But there were others that were less visible, ones that were associated with the mines themselves.

The Navajo Nation raised those concerns during a congressional hearing in 1993, citing the physical hazards (shafts, pits, and debris piles) and the potential exposure to uranium ore (contaminated materials, and heavy metals), according to the Navajo Nation Environmental Protection Agency’s website.

Fifteen years later, in June 2008, following the completion of various studies, several federal agencies finalized a five-year plan for cleaning up the legacy of uranium mining on Navajo land, the Navajo Nation EPA’s website says.

The GAO acknowledges the lingering hazards in its May 2014 report:

“… the Navajo people continue to live with the environmental and health effects from mining operations: more than 500 abandoned mines are located across the reservation, some close to homes and communities, and an unknown number of homes and drinking water sources contain radioactive elements. According to the Environmental Protection Agency (EPA) and other federal agencies, health effects—including lung cancer, bone cancer, and impaired kidney function—can result from exposure to elevated levels of uranium and other radionuclides.”

The GAO’s May report is the latest to track the federal government’s efforts to locate and clean up the abandoned mines, including a recent five-year plan in which several federal agencies coordinated their resources and had some success, according to the GAO:

Reasons agencies met the targets were primarily because additional federal and other resources were dedicated to these efforts compared with prior years. For example, from 2008 through 2012, EPA spent $22 million to test and replace contaminated houses, compared with $1.5 million spent in the preceding 5 years.

The GAO report makes clear, however, that there are significant challenges to a successful remediation effort, including money:

Federal agencies face a variety of challenges in continuing to address uranium contamination on or near the Navajo reservation. For example, according to EPA officials, funding for EPA’s efforts at the Navajo abandoned uranium mines is expected to decrease from funding levels available during the 2008 5-year plan because of overall declining federal resources for cleanup. Further, agencies face challenges in effectively engaging tribal communities, in part, because agencies have not always collaborated on their outreach efforts.

Of course, there are bright spots in the still-evolving story. This spring, $1 billion was set aside to clean up mines on the reservation. According to the Associated Press, the money is part of a $5.15 billion settlement that the federal government reached with Anadarko Petroleum Corp. for the cleanup of thousands of long-contaminated sites nationwide.

But the more than $1 billion will address about 10 percent of the tribe’s inventory of abandoned uranium mines, the news service reported.

That means, more than 30 years after the spill, the majority of mines await clean up while thousands of Navajos lead lives potentially in harm’s way due to the legacy of uranium mining on the reservation.