The Wisconsin scientist who first discovered mini tsunamis on the Great Lakes now says those "meteotsunamis" are a major cause of rip currents, the underwater backwash of waves known to pull swimmers away from shore.

Chin Wu, professor of civil and environmental engineering at the University of Wisconsin-Madison, headed a team that studied what caused the July 4, 2003, events that spurred seven drownings on Lake Michigan.

Unlike ocean tsunamis caused by geological action, namely earthquakes, meteotsunamis are caused by weather, namely wind. Meteotsunamis are a single wave as opposed to a wind-whipped series of waves. They are generally fairly small, nothing like ocean tsunamis.

The study's findings, published Feb. 14 in the journal Scientific Reports, said the 15-minute windstorm on July 4, 2003, formed a moderate-height - less than 4 inches - meteotsunami, which is what eventually caused the unexpected rip currents that took seven lives near Warren Dunes State Park several hours after the storm passed.

Wu said rip currents formed by the one-time wave of a meteotsunami can last for hours afterward, so beachgoers need to be cautious even after a storm passes.

"Please do not go back to the beach because that's the most dangerous time. The water might look calm, but underneath, there's a hidden danger," Wu said.

There were 117 drownings last summer across the Great Lakes - the highest number ever reported, according to the Great Lakes Surf Rescue Project.