– Monday was the perfect beach day in Chicago. It was hot, the sun stung and Markius Woods was itching for a dip in the lake. He begged his younger sister to join him at 31st Street Beach.

“No,” Markeishaa Woods recalled telling him firmly over the phone, just 15 minutes before the accident. “I don’t swim in water when I can’t see the bottom of it. I don’t trust it.” The most she ever dares to do is wet her toes in the white foam. Besides, she had just heated up dinner.

While all the Great Lakes can be dangerous, statistics point to Lake Michigan being the deadliest. It has had nearly as many drownings and water rescues since 2010 as the four other Great Lakes combined.

And drownings in Lake Michigan have surged so far this year with at least 20 recorded, compared with 23 for all of 2015, according to data compiled by the Great Lakes Surf Rescue Project. The nonprofit, which promotes water safety awareness, bases its drowning tabulations on reports from the U.S. Coast Guard, first responders and news outlets, and includes some unconfirmed drownings.

Experts say the distinctive shape of the lake, which is 307 miles long with parallel, uninterrupted shores running north to south, makes it susceptible to dangerous currents. Lake Michigan is also the Great Lake with the sandiest shores, drawing more visitors and creating tides along sandbars that are deceptively strong and prone to risky currents.

“People go out there, unaware of the risk. Then those waves come and beat you up. They’re relentless, and that’s something that’s radically different from what the ocean has,” said Mark Breederland, an educator with the Michigan Sea Grant Extension. “Oceans have long, periodic waves. Our waves are every three seconds. Here comes another one and another one and another one.”

Since 2010, Lake Michigan had at least 223 drownings, nearly as many as Lake Huron, Lake Superior, Lake Erie and Lake Ontario combined, which together had at least 251 drownings, the rescue project said. About 15 of those Lake Michigan drownings occurred in the Greater Chicago region, with three, including that of Woods, taking place in the past week.

David Halford, 37, of Hobart, Ind., and Kyle Reibly, 26, of Hammond, died Saturday in Gary, Ind., trying to help others struggling in the surf, the Indiana Department of Natural Resources said.

“We’re a tremendous tourist area. Lots of people utilize Lake Michigan, but they’re not familiar with the kind of waves that exist out there and really how hard it is to provide help if someone gets in trouble,” Breederland said.

Lake Michigan had 83 current-related drownings and 253 water rescues of swimmers caught in currents between 2002 and 2015. The total current-related incidents are more than double the number of incidents for the four other Great Lakes combined, which had 69 drownings and 68 rescues related to currents, according to data compiled by the National Weather Service and Michigan Sea Grant.

The data, which only tabulate confirmed current-related drownings and may be incomplete because of underreported rescues and limited access to Canadian numbers, shows that Lake Michigan averages about six current-related deaths per year.

The lake’s unmatched drowning toll is the result of its shape and formation, making it favorable for weather and wave conditions that create hazardous currents, experts said. Winds blowing north to south over 307 miles of open water, with no trees or obstructions to interfere, allowing waves to build before they crash onto the shore, said Dave Benjamin, executive director of the rescue project.

Strong winds can form high waves, forcing large amounts of water to hit the shore. That water then needs to find a way to flow back into the lake, and by doing so generates dangerous rip currents, he said. The lake also often experiences longshore currents, formed by winds hitting the shoreline at an angle.

Lake Michigan’s high volume of beaches puts swimmers at greater risk, too. And sandbars, the sand that piles up parallel to the beach, give people the perception of relief, since they can touch the sand. But when people swim across the sandbars, farther and farther from the shore, they’ll suddenly find themselves in water up to their neck, no longer able to touch the sand. And when large waves come unexpectedly, swimmers are at risk of drowning, Breederland said.