– After a year of scouring the depths of Lake Michigan with a sonar-equipped fishing boat, Steve Radovan finally got a hit on the gray-scale monitor in the captain's cabin in May 2016.

The 71-year-old shipwreck enthusiast powered down the Discovery's engines and dropped a waterproof camera attached to a rope into roughly 300 feet of water. The images revealed a three-masted barquentine, covered in mussels and algae but lying on the bottom still largely intact. After reporting the finding to the state of Wisconsin, he learned that the foundered ship was the Mojave.

With a cargo of 19,500 bushels of wheat, the ship had set sail from Chicago en route to Buffalo in 1864. The Mojave was spotted by the crew of a passing ship as it dropped into a trough of stormy waters. The vessel lay deep below the surface for over a century.

"This is the stuff the moviemakers dream of. This is just like it was when it sank to the bottom," Radovan said with a grin, watching the images from his home office. "No human has seen this ship since 1864."

For more than a century, sinking ships claimed thousands of lives, burnishing Lake Michigan's reputation as being among the most dangerous waters to navigate. Its notoriety as the deadliest of the Great Lakes is evident from an expansive graveyard of shipwrecks spanning the shoreline of Wisconsin — a testament to the perils taken on by crews and passengers who navigated the waters in the 19th and 20th centuries.

Under a new push by the National Oceanic and Atmospheric Administration, the ghostly collection of sunken vessels could become the first national marine sanctuary in Lake Michigan and the second in the Great Lakes. NOAA is expected to make a decision by next year, then Wisconsin Gov. Scott Walker and Congress will review the plan.

Advocates say time is of the essence if the public is to view and study the wrecks because their structural integrity is endangered by the zebra mussels, an invasive species known for clinging to objects underwater and rapidly reproducing. The mussels can be cancerous, as evidenced by what happened to the Gallinipper, a fur trading ship that went down in 1851 and remained in pristine condition on the lake floor for more than a century.

"If it was raised, it could sail again," said Brendon Baillod, a maritime historian. "But it became so encrusted and caked in zebra mussels it started to collapse. So, in a sense, there's an urgency to finding these wrecks now, because in 10 years, they could start disappearing."

While the sheer number of sunken vessels makes Wisconsin's slice of Lake Michigan stand out, the site is also renowned for the remarkably sound condition of many downed ships. Fifteen wrecks known to researchers are virtually intact, and 18 are listed on the National Register of Historic Places, per preliminary reports. Divers have found many with masts still standing, unbreached hulls, and even one with nautical charts still stowed in the wheelhouse — something that would be unlikely in ocean waters.

"Cold, fresh water," said Russ Green, NOAA regional coordinator. "The fact that it's salt-free helps preserve iron and wood, and the cold water is like a big freezer that acts against deterioration."

The proposed 1,075-square-mile site offshore of Manitowoc, Sheboygan and Ozaukee counties contains 37 known shipwrecks dating from the 1830s through the early 1900s. Researchers say the area could be home to as many as 80 other undiscovered wrecks.

"These shipwrecks really tell us the history of how shipping was the engine of the American economy," Green said. "There's a huge legacy of risk, sometimes tragedy, personal stories of innovation, entrepreneurship — all locked into this proposed area."

The explosion of zebra mussels that threatens to destroy the sunken vessels has, ironically, made it easier to discover and explore the wrecks. One zebra mussel can filter a liter of water a day, so once-plentiful microorganisms like plankton, which clouded the waters, have been decimated.

Since the introduction of zebra mussels in 1990, underwater visibility that was once 5 to 10 feet is now 80 to 100 feet. It isn't unusual for one or two shipwrecks to be found each year, some spotted by satellite imaging or low-flying aircraft.

With the improved water clarity, the Wisconsin Historical Society wants to help more people view the wrecks by establishing a water trail of shallow-water shipwrecks that can be seen by paddle boarders, kayakers and snorkelers.

On a recent afternoon, Tamara Thomsen, a state maritime archaeologist, prepared to survey the J.M. Allmendinger, a wooden steamer that ran aground near Mequon where it was eventually pulverized by waves. Seated on the edge of her Boston Whaler in a heavy-duty dive suit, she took a couple of airy breaths from her oxygen tank, placed one hand over her goggles and the other atop her head, and fell backward into the water.

About 15 feet beneath the surface, a tape measure lay atop the ship's skeleton, stretching 94 feet across the lake bottom. The assemblage of wooden planks was speckled with zebra mussels and fuzzy, green algae. Nearby, a long, slender boiler that once powered the vessel lay on the rocky lake bottom.

"It's like Pick-Up Sticks shipwrecks here," Thomsen said. "You're trying to figure out what it was before. Then, you're like, 'Oh, I get it. Those are the walls where the deck collapsed and slid over here.' "

For hours at a time, Thomsen photographed the wreckage as other divers drew what they saw on underwater slates and scrawled measurements in hopes of creating a scaled rendering of the ship. The Historical Society's work catalogs shipbuilding practices that were unique to the Great Lakes and have been lost over the generations.

The most common ships in the potential sanctuary are 19th-century schooners — nimble sailboats with two or more masts, similar to a classic pirate ship.

Many times, it's the contents of a ship that help identify and tell its story. After the Wisconsin Department of Natural Resources detected a steel steamship about 16 miles northeast of Port Washington, maritime archaeologists discovered hundreds of antique cars in the hull — identifying it as the Senator, a car ferry.

Navigating through dense fog, a passing ship rammed the Senator, sending its crew and cargo of 268 Nash automobiles to the lake bottom on Halloween 1929, days after the historic stock market crash.

It was these kinds of stories that hooked Radovan. He's part of a fraternity of boaters with pricey radar or sonar equipment who continue to navigate historic shipping routes in search of wrecks.

Radovan said he has found six shipwrecks independently and nine with others. He has his sights set on at least two others in the region.

An array of shipwreck antiques are on display at his home: a table made from the hatch of the Prins Willem V, a wheel made from the Roscinco and a steam gauge from the SS Pewabic. As a young man, Radovan and his colleagues were eager to bring items up and show them off to the local newspapers to spread the news of this lost history, he said.

Others, however, were taking items out of the water for profit. Some wrecks went down before the advent of paper money and contained gold and silver coins. Wisconsin statute eventually prohibited the removal or disturbance of items on sunken ships, which, in some cases, includes human remains.

"These shipwrecks were being stripped," Radovan said. "Everything that wasn't nailed down was taken. All of these things are basically time capsules. And this was history, but it was going in everyone's basement. … I'm of the philosophy let it stay where it is."

If the area becomes a national marine sanctuary, federal regulation would prevent other unintended damage by prohibiting grappling and anchoring at shipwreck sites.