Winslow Dumaine was heading to a store on Chicago’s North Side when he saw it: a hole in the sidewalk on Roscoe Street with an uncanny resemblance to a rodent.

Mr. Dumaine, who is an artist and comedian, said the hole represented two themes often present in his work: morbidity and whimsy.

“Had to make a pilgrimage to the Chicago Rat Hole,” he wrote in a social media post this month, including a close-up photo of the concrete cutout.

The post, which has since been viewed five million times, inspired an untold number of Chicagoans to make their own excursions to a quiet residential area of Roscoe Village, a neighborhood known for its cozy taverns, independent boutiques and old-fashioned bakeries.

People have started making offerings to the mysterious, fat-rat-size crevice: candles, coins, flowers, a small tomb with a photo of a rat, and a bag of cinnamon rolls from Ann Sather, the beloved Chicago restaurant chain.

Both online and off, the “Chicago rat hole” became a shared joke in a city that prides itself on its sense of humor; passers-by giggle at the miniature memorial, pausing to talk to other visitors and take pictures of themselves at the hole. And in the city that was recently declared the “rattiest” in the United States — deemed to have the worst rat infestation by the pest control company Orkin — for the ninth consecutive year, Chicagoans have reveled in the symbolism.

Even a local politician, State Representative Ann Williams, got in on the joke.

In a video posted to social media on Wednesday, she touted the attractions of the district she represents, including the many bars and restaurants, Wrigley Field, “and, of course, the Chicago rat hole,” she said, as the camera panned down to the sidewalk.

At the rat hole on Thursday, a toddler in a pink fleece jacket gleefully prodded a small toy mouse that had been placed in its center — the latest offering.

As Mr. Dumaine pointed out the tiny claw marks in the concrete, Jenny Morales and her daughter, Janelle, approached, laughing.

“It’s not every day you get to see a rat hole,” Jenny Morales said. “It’s a cold winter day so I just figured we’d just come see something.”

“Just see the rat hole!” Lora Bothwell, the owner of a nearby day care, interjected in the style of a carnival barker. “I walk kids past here all day every day, and we always talk about ‘Is it a rat? Is it a squirrel?’”

Personally, Ms. Bothwell thinks it’s a squirrel: “I don’t think a rat would jump and splat like that,” she said.

Mr. Dumaine agreed that the shape of the imprint was not, in fact, so rat-like.

“It has the big hips of a squirrel,” he said, “but ‘Chicago Rat Hole’ is just a great band name.”

The hole’s origins are unknown but have been debated online, in local media and at the site itself.

However it happened, and whichever animal species may have been involved, it is at least 20 years old, said Ms. Bothwell, who has lived near the spot for 27 years. She said former clients were texting her with delight at the appearance of the hole on social media and local news.

Since moving to Chicago from his hometown, Omaha, in 2017, Mr. Dumaine has often posted pictures of signs and other symbols of urban arcana that he finds interesting or funny, but the rat hole post has blown them all away.

The widely viewed post has given Mr. Dumaine’s art — including hand-drawn tarot decks and irreverent T-shirts and makeup bags — a huge boost, he said. But while he quickly claimed a local Fox News affiliate’s description of him as the “rat hole guy” in a recent TV story, he said he had turned down offers to develop merchandise and profit directly from it.

“I refuse to take any authority over it. I want it to be for everybody, I’m not colonizing a rat hole.”

The rat hole is a public good as far as Mr. Dumaine is concerned, just another example of animals leaving their mark on human civilization, like the cat paw prints found on a 2,000-year-old Roman roof tile, or the inky paw prints found on a 15th-century manuscript.

In his artwork and comedy, Mr. Dumaine tries to play off the zeitgeist and make something funnier or more extreme. But, he said, the rat hole eludes such treatment.

“I can’t gild the lily,” he said. “I can’t make this funnier than it is.”