Almost every waterway in America has a story of aquatic monsters lurking in the dark, beneath a river's waves or a lake's surface.
Bernie Kuhajda, an aquatic conservation biologist with the Tennessee Aquarium's Conservaton Institute, said stories abound of divers poking around beneath dams and discovering catfish as big as Volkswagens.
"Every dam has a story like that," he said.
But which of these legends are true and which are just tall tales?
Dr. Zeb Hogan, with National Geographic, set out on a journey to find out if these fish of epic proportions are real or just groundless rumors.
"We're investigating these stories that have been around for a long time of these car-sized catfish," Hogan said, who hosts the National Geographic WILD show "Monster Fish," whose fourth season is set to air July 5.
The "Monster Fish" crew recently visited the Chattanooga area for a look at some of the Tennessee Aquarium's river giants to compare fact and fiction.
The aquarium's River Giants exhibit showcases some of the world's biggest fresh-water fish, from the giant blue catfish to lake sturgeon and freshwater sting rays.
"The bulk of the fish in there are river giants or mega fish," said aquarium curator of fish Thom Demas.
A mega fish is anything that reaches 6 feet in length or greater.
Hogan was especially interested in the exhibit's giant blue catfish, the possible culprit behind the rumored monster fish.
"Almost everyone we talked to thought the stories of car-sized catfish were true. So the perception is that they're real," Hogan said.
But the aquarium's specimen was only between 4 and 5 feet long. The fish rarely get above 6 feet these days, Kuhajda said.
"That's not quite the same size as a Volkswagen," he said.
Historical records, however, indicate that these river giants really did reach legendary proportions. So what happened to the goliaths of America's rivers?
"It's still a little bit of a mystery ... because one of those hasn't been caught for a while. They're not getting that big anymore, and we want to know why," Hogan said.
Demas said it was directly related to environmental factors and consumption patterns. Overfishing and pollution have limited many fish to much smaller sizes than their ancestors.
Given the right conditions, Demas thinks the fish would surpass the 6 or 7 foot limit.
"They could all probably grow to their genetically predisposed sizes," he said.
Hogan said he hoped the show would make people aware that many of the world's monster fish are in danger of disappearing forever.
"It's a lot about fishing, but it's also about making sure we have these fish for the future," Hogan said.
But not all is lost. The Tennessee Aquarium has a conservation project to bring the lake sturgeon back to the Tennessee River.
The lake sturgeon, a species native to the area, disappeared from local waterways around 1960, when water quality and river dams made living conditions too harsh for them to survive.
But since the program started in 2000, more than 125,000 lake sturgeon have been released into local waterways.
"The end goal is to have a self-sustaining population," said Thom Benson, senior marketing and communications manager at the aquarium.
Hogan hopes conservation programs like this one will safeguard these monster fish for future generations, and that one day, he'll get to see a true giant.
"I've seen some big fish, but I've always had a dream for me to see some bigger fish," he said.
Contact staff writer Lindsay Burkholder at 423-757-6592 or firstname.lastname@example.org.