Beneath the cold waves of Lake Michigan rests an aging shipwreck, its wooden planks encrusted with brown-and-gray zebra mussels, that may be the remnants of a 17th-century ship called the Griffin, two Michigan-based treasure hunters say.

French explorer René-Robert Cavelier, Sieur de La Salle built the Griffin in 1679, but it was lost in Lake Michigan the same year.

In 2011, Michigan-based treasure hunters Kevin Dykstra and Frederick Monroe found a shipwreck as they were searching for the $2 million in gold that, according to local legend, fell from a ferry crossing Lake Michigan in the 1800s, they told WZZM, a western Michigan news station.

Their sonar caught a mass below, and Dykstra dove into the water to take video.

"I didn't go down there with the expectation of seeing a shipwreck — I can tell you that," Dykstra told Live Science. When he and Monroe later reviewed the video, they realized it might be the Griffin. [Shipwrecks Gallery: Secrets of the Deep]

But other experts aren't convinced that the wreck is the Griffin. Rather, it may be the remnants of a tugboat that was scrapped after "steam engines became more economical to operate," said Brendon Baillod, a Great Lakes historian who has written scholarly papers on the Griffin.

The wreck's discoverers agree that more evidence is needed.

State archaeologists reviewed the footage, and "They've been very diligent to say, 'This is really interesting; these are some neat pictures,'" Dykstra said. "Can we call this the Griffin? Certainly not — not without a lot more information — but these are very compelling."

Le Griffon

La Salle sailed the Griffon through the Great Lakes and crossed into Lake Michigan in an effort to reach the mouth of the Mississippi River, Baillod said. But the explorer ran out of money, so he disembarked with the other expedition leaders, leaving the ship and its crew to pay off his debts with furs. La Salle never saw the Griffin again. [Disasters at Sea: 6 Deadliest Shipwrecks]

La Salle returned to the area in 1682, to try again to locate the Mississippi's mouth. But members of the Potawatomi tribe brought pieces of the ship to the explorer, including some moldy beaver furs and a pair of sailor's britches, said Baillod, who translated La Salle's journal from French to English.

The Native Americans told La Salle the crew planned to sail toward the Straits of Mackinac in stormy weather.

"The [American] Indians told the captain not to sail out, to wait the storm out, but he wouldn't listen to them," Baillod said. The captain lost control of the ship as strong winds blew it away from shore, southward, toward islands in the distance.

"They lost the ship from sight," Baillod said, "and that's the last anybody has ever seen the Griffin."

Shipwreck spotted

About 30 adventurers have claimed to have found the Griffin, usually by happenstance, Baillod said.

"They're looking for something else, they find an old ship and they've heard of the Griffin, so they pronounce it the Griffin," Baillod said.

Moreover, Baillod said he hasn't heard of anyone looking for the Griffin near the Beaver Island archipelago, which is likely the area mentioned in La Salle's journal, Baillod said.

But the latest finding, made popular again by Wreck Diving Magazine in its latest issue, holds a number of clues about the ship's past.

"There was no rudder on the boat," Dykstra said. "That was kind of telling to us that the ship probably weathered a storm; otherwise, there would probably be a rudder on it."

They also found a part of the ship that they said could be a mussel-covered griffin, the mythical beast carved onto the ship's bow.

On a subsequent dive, Dykstra took a magnet with him to help determine the metal composition of the ship. Unexpectedly, a nail attached itself to the magnet, and the treasure hunters only discovered it later, once they were above water.

"When we had it looked at, they [the archaeologists] could tell that the nail was very old," Dykstra said. "It was a hand-forged nail, which helps date it back to that time period, we feel." [In Photos: Arctic Shipwreck Solves 170-Year-Old Mystery]

The state of Michigan has rules stipulating that artifacts found on state land, including the land at the bottom of the Great Lakes, are state property. The two men did not bring up the nail on purpose, and they plan to return it to the state, said Dean Anderson, the state archaeologist for Michigan.

About 1,500 shipwrecks have been found on the bottom of Lake Michigan, Anderson said, and it's unclear whether this one is the Griffin.

"It's very difficult to access a wreck based on photo and film footage," Anderson said. If the state underwater archaeologist were to look at the wreck, he would look for artifacts that could be dated, such as ceramics or glass.

Unsolved mystery

Baillod said he is "99 percent sure" that the wreck is not that of the Griffon. The figurehead likely isn't the remains of a griffin, he said, but a "big encrustation of zebra mussels," on burned wood. He noted that the wreck is near the western Michigan coast, not near Beaver Island, the area mentioned in La Salle's journal.

But Dykstra and Monroe said they'll wait until they hear the final word. They're not going back to the wreckage for a while, so they don't make the site vulnerable to other treasure seekers. In the meantime, the duo plans to continue their hunt for the gold bullion.

"It's a mystery ship that got in our way," Dykstra said, "and now, we're going for the gold."

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