HOUGHTON — It was a beautiful July 4 at Warren Dunes with sunny skies and calm water perfect for swimming. Seven swimmers drowned that day.

The 2003 incident was at first attributed to rip currents but, with calm waters, where did they come from?

Chin Wu, a civil and environmental engineering professor out of the University of Wisconsin-Madison, may have found the answer and shared his research earlier this week at Michigan Technological University.

Wu thinks a little known Great Lakes phenomenon called a Meteotsunami was behind the tragedy. Meteotsunamis are common occurrences across the globe that are still not fully understood. Unlike a traditional tsunami, which is caused by seismic shocks, a meteotsunami is caused by weather, Wu explained. Specifically the wind and pressure disturbance from a fast-moving storm.

Just like the storm that passed a few hours before the drowning incidents at Warren Dunes.

The resulting wave can be up to six feet though most are small. Around 100 occur in the Great Lakes each year.

The meteotsunami itself is a little-known danger to beach-goers but Wu is even more concerned about the pullback and very strong currents that follow, which he discovered in a recent breakthrough while studying the Warren Dunes case.

Thanks to data forensics, Wu was able to determine a meteotsunami did occur on that July 4. It was not very large, only around 0.3 meters or just under a foot, but it was enough to create rip currents hours later as the water pulled away.

By that time, the storm was long gone and many beach-goers were back in the water. The unexpected nature of these rip currents is why Wu considers the period after the meteotsunami hits to be the most hazardous. As the storm and the later arriving wave roll into shore, beachgoers are not typically out swimming. However, despite the water still being turbulent hours later from that pressure, swimmers may return.

Wu hopes to educate people on this new discovery and the danger of meteotsunamis to help beach-goers stay safe.

“(If) a storm comes, a meteotsunami comes. Don’t go back, please. Even if you (see) very calm water, people don’t go back,” Wu urged.