Swimming in Onondaga Lake is something most residents of the nearby city of Syracuse would like to avoid.

For over 100 years, the lake was used as a dumping ground for the county and industry groups like Allied Chemical, whose factories line the shore. It became known as the most polluted lake in America, and its reputation hasn't faltered.

But over the past decade, the lake’s main polluters have spent more than $1 billion on water treatment plants and greywater infrastructure, as well as an ambitious engineering effort to dredge and cover 16 percent of the contaminated lake bottom.

They’re calling the cleanup a success, but not everyone is convinced.

“It's still a Superfund site,” says Tadodaho Sid Hill, chief of the Onondaga Nation. “There are still toxins in there.”

Superfund sites are areas so polluted with toxic waste that they pose a risk to human health and the environment. There are 1,343 active Superfund sites in the U.S., and a disproportionately large number of them are on land owned or claimed by Native Americans.

Onondaga Lake, which obtained its Superfund designation in 1994, is no exception. The lake is central to Native American history, says the Onondaga Nation. It was on its shores that the Iroquois Confederacy — a democratic treaty between five warring nations, a constitutionally recognized precursor to the U.S. Constitution — was formed.

One of the toxins that concerns Sid is mercury. Allied Chemical, later renamed Honeywell, dumped 165,000 pounds of mercury into the lake, leading to a complete fishing ban in 1970.

To clean it up, Honeywell created a remediation plan that included building an underground barrier wall to keep contaminated groundwater from seeping into the lake, dredging and covering 15 percent of the contaminated lake bottom, and treating the water. It has also spent millions to restore the vegetation. Because of this, the state says the lake is now clean.

“We implemented the cleanup plan that was identified by the agencies,” said John McAuliffe, who oversaw Honeywell’s cleanup plan. “And I think we went beyond that because we spent a lot of effort in identifying the habitat that community wanted to see around Onondaga Lake.”

But even though Honeywell’s cleanup plan significantly reduced the lake’s mercury levels, the fish in the lake still contain levels that could harm humans if ingested. So the state continues to recommend that children and women of childbearing age avoid eating fish from the lake altogether.

VICE News went to Onondaga lake to examine the debate that’s pitting the county and Honeywell against local residents who think the lake cleanup has barely begun.