Even from afar, the snapping turtle perched on rusted chains in the Chicago River looked gargantuan.

Kayakers enjoying a spring outing recorded the plump reptile, marveling at its wrinkled and chunky legs and its shell, which barely covered its thick green body.

In the video, which was posted on Twitter this month, one of the kayakers, Joey Santore, sounding astonished, cries with an expletive: “Look at the size of that thing!”

Mr. Santore’s friend, Al Scorch, gave the turtle a name befitting such an enormous reptile: Chonkosaurus.

Ever since, thousands of people online have heaped admiration on Chonkosaurus, writing about the beefed-up turtle and about the men whose wonderment has brought the video more than 700,000 views.

Mr. Santore, who posted the video on Twitter and grew up in a Chicago suburb, said in an interview that he and Mr. Scorch were on the river on May 5 to film a video for his YouTube channel, “Crime Pays But Botany Doesn’t,” which educates viewers about ecology.

“I get fans of the channel that like the blend of cynical social commentary muttered through a Chicago accent and then blended with plant education,” he said.

At first, Mr. Santore and Mr. Scorch couldn’t quite make out what was sitting above the water. Then they paddled closer.

“It was so fat, you know, the legs kind of spilled out,” Mr. Santore recalled. “It looked like a fat sandbag.”

Its size has prompted a question online: What has the animal been eating?

One person’s guess was simple: anything it pleases. Another guessed beef and sausages. And some, alluding to “Teenage Mutant Ninja Turtles,” guessed pizza.

Perhaps the most valuable insight came from the men in the video who actually saw Chonkosaurus.

“Holy hell, you look good!” Mr. Santore says in the video. “I’m real proud of you. You’ve been eating healthy?” He asks the turtle if it has heard of liquid salad, and Mr. Scorch later says that Chonkosaurus is “thick but strong.”

Chonkosaurus’s nutritional pursuit does not appear to be completely selfish, however.

Chris Anchor, a senior wildlife biologist with the Forest Preserves of Cook County, said that the turtle is female — and most likely “loaded with eggs.”

On the day the video was taken, he said, Chonkosaurus might have been sitting on a bunch of bollards and chains because she wanted her eggs to warm up and mature.

The turtle is probably about 50 years old and has finished hibernating, Mr. Anchor said.

While he did describe Chonkosaurus as “very large,” the snapping turtle is not among the largest he’s seen. Mr. Anchor, who sees around 300 to 500 turtles every year through his work, said the species can weigh as much as 60 pounds.

Though it’s difficult to tell in the video, Mr. Anchor guessed that Chonkosaurus weighed around 40 pounds.

What was more noteworthy, he said, was that the video highlighted the health of the Chicago River and surrounding land.

Mr. Anchor recalled that, when he was growing up in the 1960s, “the river was an open sewer” with only a handful of species of fish and no recreational kayaking.

After the passage of the Clean Water Act in 1972, the water quality of the Chicago River improved substantially. Now, there are about 30 species of fish in the river, and people enjoy being on the water.

“It’s something that would not have occurred 40 years ago,” Mr. Anchor said. “You’ve got a lot more eyes in the river now that are seeing things that no one ever noticed before.”

It is hard to tell what exactly Chonkosaurus managed to snap into her belly. Large snapping turtles have been known to grab ducks on the river, drag them underneath and rip them apart in murky waters.

Particularly huge snapping turtles are also capable of ripping up deer or raccoon carcasses with their claws.

The diet of Chonkosaurus remains a mystery, but that has not slowed the online compliments and veneration. One Twitter user summed up these sentiments: “All hail Chonkosaurus.”