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Residents and tourists in Chicago did a double take Saturday morning as mysterious fins sliced through the bright green water during one of the city’s most beloved traditions.

Reports of Sharks in the Chicago River spread quickly during the annual river dyeing celebration, leaving many wondering if the festive green waters had suddenly become the stage for an unexpected aquatic predator.

Strange Fins Circle Emerald Waters

The unusual sightings happened during the city’s famous Chicago River dyeing, when crews transform the Chicago River into a brilliant shade of green each year.

At around 10 a.m. Saturday, spectators spotted what looked like shark fins cutting through the glowing emerald current.

The fins appeared to circle the water repeatedly, fueling speculation that something far more exotic than fish might be lurking beneath the surface.

For kayakers and boaters enjoying the event, the thought of actual Sharks in the Chicago River would have been the ultimate surprise.

Netflix Reveals the Truth Behind the Stunt

Fortunately for those paddling through the river, the fins were not real.

They were part of a visual stunt organized by Netflix to promote an upcoming thriller film titled THRASH.

According to a Netflix spokesperson, the staged shark fins began appearing along the river’s main stretch starting near Columbus Drive Bridge.

The promotional display moved up and down the waterway between 11:30 a.m. and 4 p.m., creating the illusion of sharks circling through the city’s bright green river.

The unexpected spectacle quickly caught the attention of spectators and passersby, turning the viral moment into a buzzworthy marketing stunt.

Film “THRASH” Brings Sharks After a Monster Storm

The film at the center of the stunt, set for release on April 10, explores a dramatic disaster scenario.

In THRASH, a Category 5 hurricane devastates a coastal town. The storm’s massive surge pushes seawater—and hungry sharks—deep into populated areas, turning flooded streets into dangerous hunting grounds.

The film was written and directed by Tommy Wirkola and features a cast including Phoebe Dynevor, Whitney Peak, and Djimon Hounsou.

A Viral Moment During a Beloved Chicago Tradition

The annual river dyeing event is one of Chicago’s most recognizable celebrations leading into St. Patrick's Day, drawing thousands of spectators to the city’s downtown riverwalk and bridges.

This year, however, the tradition gained an unexpected twist as rumors of Sharks in the Chicago River rippled across social media.

In reality, the only predators in the water were cleverly engineered props—proof that sometimes marketing can be as theatrical as the movies it promotes.

Still, for a few suspense-filled hours, the emerald river looked less like a festive landmark and more like the opening scene of a blockbuster thriller.