Listen and subscribe to our podcast: Apple Podcasts | Spotify

The frogs were all wrong.

Located in a small pond in Henderson, Minn., the amphibians discovered by a middle school class had legs growing out of legs, missing eyes, bent spines and contorted jaws. Their abdomens and backs sprouted extra limbs, reaching out to nothing in the air.

It was horrifying, Judy Helgen remembered. Helgen was the first state scientist to inspect the pond in 1995, after the kids called in their discovery.

"They had buckets and buckets of these horrific frogs," said Helgen, a retired biologist with the Minnesota Pollution Control Agency. "All of them were young, because they couldn't survive long."

The discovery quickly became national news. More and more reports of deformed frogs came in from across the state, the Midwest and the country. Wherever people looked, they seemed to find frogs with extra limbs, missing eyes, tails and bodies that looked half-melted away.

A deformed frog with an extra leg that was found in Minnesota.
A deformed frog with an extra leg that was found in Minnesota.

Minnesota Pollution Control Agency via Associated Press

Nearly three decades later, some Star Tribune readers are still wondering what happened. One reader turned to Curious Minnesota, a reader-driven project that answers questions about the Gopher State, to ask if a cause was ever found for the malformations. Another reader asked if road salt or a company's pollution were to blame.

There are several theories about what caused the deformities — including ultraviolet rays, parasites, farming chemicals and industrial pollution. But no definitive answer was ever found.

"Nobody knows the cause of a surprisingly large percentage of human defects, let alone frogs and amphibians," Helgen said. "It gets really complicated."

'Bad things were just everywhere you looked'

Helgen first encountered deformed frogs in 1993, two years before the big discovery in Henderson's Ney Pond. A boy in Granite Falls was collecting the critters in his backyard to sell as fishing bait when he came across three or four with the same missing or extra limbs. Helgen and a small team of scientists started monitoring wetlands around the state.

She found leaking pipelines and computer "bone yards" — where people would burn old electronics at night — that sent mercury, lead and other toxins straight into the water. In some homes, she witnessed tap water light on fire.

"Coal-fired power plants just discharged their ash straight into rivers," she said. "Bad things were just everywhere you looked."

Every species is prone to deformity. In frogs and amphibians, malformations typically happen to between 1% and 2% of the population.

In the Henderson pond, the kids found dozens of frogs. More than half them had missing limbs and eyes, branching legs or other problems. The sheer number of malformations was the clearest sign that whatever caused the problem wasn't natural, Helgen said.

Biologist Judy Helgen, right, collected water samples from a pond near Henderson, Minn. in 1997.
Biologist Judy Helgen, right, collected water samples from a pond near Henderson, Minn. in 1997.

Tom Sweeney / Star Tribune

The pond is surrounded by farmland about 55 miles southwest of Minneapolis. She and other scientists started investigating farming chemicals and pesticides.

Frogs typically survive a Minnesota winter in one of three ways. Some burrow down and bury themselves a few feet into the soil. Some spend all winter underwater in a river or lake — drawing oxygen through their skin. Others just let themselves freeze where they stand, on the forest floor or along the bank of a river, and essentially reanimate during the spring thaw.

That meant that in addition to the area surrounding the pond, the contamination could be coming from rivers, lakes and soil where mature frogs spend the winter, Helgen said.

She chronicled the difficulty of blaming a single chemical for the deformities among the cocktail of possible pollutants in her 2012 book, "Peril in the Ponds: Deformed Frogs, Politics, and a Biologist's Quest." And the problem wasn't limited to Minnesota.

"They were found in 42 states, Canada, Japan, Denmark, Italy, Russia," Helgen said. "It was all over. It was an international problem."

The EPA and U.S. Geological Survey funded studies, hoping to find answers. Experts were called in and theories raised. Egos and politics clashed. A few years passed, and the funding stopped. Reports of malformed frogs largely disappeared from the public consciousness.

Other scientists posed theories. Parasites are known to cause certain types of deformities in frogs. It could be possible that farming fertilizers spurred an unnatural bloom of the parasites, some argued.

But the parasites in question were never found in the Henderson pond and many of the other outbreak sites, Helgen said.

Lab tests showed that ultraviolet rays could also cause amphibian deformities. But studies were conducted that cast doubt on whether UV rays could penetrate deep enough into a frog's eggs or skin in the wild.

Frog deformities are no longer as common

Deformed frogs are still found today, but not to the level they were in the late 1990s. Starting sometime in the early 2000s, people around the state and country stopped finding as many concentrations of the malformed creatures. The urgency and political will to find an answer vanished among other priorities.

Major cases of amphibian defects haven't been seen at Ney Pond for many years, said Becky Pollack, executive director of the Ney Nature Center, which includes the pond. The big threat to the local amphibian population now is two straight years of drought, she said.

Ney Nature Center Executive Director Becky Pollack and retired biologist Judy Helgen looked for frogs at the Ney Nature Center in Henderson, Minn. in 2012.
Ney Nature Center Executive Director Becky Pollack and retired biologist Judy Helgen looked for frogs at the Ney Nature Center in Henderson, Minn. in 2012.

Bruce Bisping / Star Tribune

"The droughts have been devastating. And now that we're going a winter without snow, we're pretty worried about the future of our pond," she said.

Little data exists on how the state's frog and toad populations are doing overall. Most frog species lay enough eggs that they can rebound relatively quickly, said Chris Smith, conservation committee chair of the Minnesota Herpetological Society.

"Frogs need both wetlands and adjacent uplands," he said. "Generally we've been seeing people get better at keeping wetlands, but the uplands get developed when people move into what was a rural area."

Helgen was never able to find a specific chemical that may have caused the deformations. But she does have her suspicions.

"I've always wondered if it might have been something that was banned or is no longer used," she said.

She pointed to cyanazine, which was sold under the brand name Bladex and had become one of the most commonly used pesticides in the country by the 1990s. The EPA banned cyanazine in 2002. The European Union and other countries phased it out around the same time. That lines up with when reports of malformations stopped becoming as common.

"But I just don't know," she said.

There are just too many potential sources to tell.

If you'd like to submit a Curious Minnesota question, fill out the form below:

This form requires JavaScript to complete.

Read more Curious Minnesota stories:

When did wild bison disappear from Minnesota?

Why do wild turkeys seem to thrive in the Twin Cities?

Why do coyotes always howl when trains go by?

Were grizzly bears ever indigenous to Minnesota?

Why don't farms water their crops at night?

How do small animals survive Minnesota's brutal winters?