The viral Chicago ‘Rat Hole’ almost certainly wasn’t made by a rat

Chicago’s viral ‘Rat Hole’ is less rat splat, more squirrel squish.

Researchers determined it was probably a squirrel that left a rodent-shaped impression in the concrete of the Windy City. Their new study, published October 15 in Biology Letters, borrowed paleontological tools to analyze the bizarre and beloved local landmark.

Michael Granatosky, an evolutionary biomechanics researcher at the University of Tennessee in Knoxville, was motivated to study the decades-old imprint shortly after artist Winslow Dumaine posted about the site in 2024. Dubbed Splatatouille, the Chicago Rat Hole quickly became a viral sensation, with visitors to the city’s Roscoe Village neighborhood leaving coins, flowers and other paraphernalia before the slab was moved to City Hall. Despite Chicago’s status as the “rattiest city in America,” the culprit has long been questioned.

“Even if people weren’t thinking of it as science, it was clear they were applying deductive reasoning,” Granatosky says. “It seemed like a fun project to apply the scientific method to something that’s lighthearted.”

Granatosky and his colleagues leveraged the viral nature of the site to amass dozens of images. From those photos, they collected measurements of anatomical features, including the snout-to-tail length and head width. Using the app iNaturalist, the team identified eight small mammal species found in Chicago. The group then compared the same measurements from museum specimens of the candidates to the imprint.

The measurements most closely match those of an eastern gray squirrel (Sciurus carolinensis) or a fox squirrel (S. niger). Granatosky says the working hypothesis is that the animal probably created the hole after it fell out of a tree that residents say was once in the vicinity. For a rat to achieve the same effect, it would have needed to be dropped by a bird, as there don’t appear to be any footprints. Rat purists note that there’s no bushy tail in the impression, but Granatosky says it’s unlikely concrete would capture such fine detail.

Even with all the information the team had, it still wasn’t able to make a definitive identification from this relatively recent critter crater, highlighting just how challenging this work can be when analyzing ancient paleontological specimens. Granatosky hopes this case study will spark debate among colleagues and public curiosity, and is currently developing a lesson plan for children.

“It’s so rare to get such a fun story,” he says. “It really speaks to what we do in my lab, which is take data and package it in a way that is accessible.”