FAIRPORT, Mich. — A wooden beam embedded at the bottom of northern Lake Michigan appears to have been there for centuries, underwater archaeologists announced Tuesday, a crucial finding as crews dig toward what they hope is the carcass of a French ship that disappeared while exploring the Great Lakes in the 17th century.

Expedition leaders still weren’t ready to declare they had found a shipwreck or the long-lost Griffin. The ship, commanded by the French explorer Rene Robert Cavelier de la Salle, was never seen again after setting sail in September 1679 from an island near the entrance of Green Bay, in what is now northern Wisconsin, with a crew of a six and a cargo of furs.

But Michel L’Hour, director of France’s Department of Underwater Archaeological Research and a shipwreck expert, said the timber appears to be a bowsprit, which is a spur or pole that extends from a vessel’s stem. It also seems to be attached to another structure below the lake bed, he said.

“All the details could be interpreted as part of a bowsprit and there’s no details which contract this hypothesis,” said L’Hour, who dove to inspect the beam with two French colleagues Monday and Tuesday. “It’s why it’s the main hypothesis now. A bowsprit which has been buried in the sediment of the lake for many centuries.”

Commercial divers overseen by scientists last week began excavating at the base of the wooden beam, hoping to determine whether it is part of the Griffin. Steve Libert, a diver and shipwreck enthusiast who has searched three decades for the Griffin, discovered the timber in 2001 and recently obtained state and federal permits to probe beneath the surface.

The beam extends 10.5 feet above the lake bed, and underwater excavators were opening a pit at the base of the post to determine whether it’s affixed to anything beneath. In another key development Tuesday, they reported that a probing device had detected a hard surface 18 to 20 feet below the lake bed. It could be a ship’s hull or deck.

“In essence, we have found a floor under that exposed wooden timber,” said Ken Vrana, the project manager. “We have more excavation to do before verifying what that surface is.”

L’Hour said the French archaeologists drew their conclusion about the beam’s age after observing differences between the section above the lake floor and the portion below the surface that the pit has exposed. The aboveground section is narrower because of erosion that must have happened over hundreds of years, he said.

Libert said he was excited by the reports and had “no doubt” the beam was part of a ship. But it remained uncertain when the team might be able to positively identify the presumed vessel.

“I think that maybe Steve found the Griffin,” L’Hour said at a briefing for reporters. “I can’t be sure, which is why I’m waiting and waiting and waiting for the proof.”

Although visibly optimistic, the searchers cautioned against expecting quick resolution of a mystery that has thrown numerous hurdles in Libert’s path.

After years of research led him to an area near Poverty Island a few miles off Michigan’s Upper Peninsula, he literally bumped into the timber during a dive. That touched off years of legal battles between his Great Lakes Exploration Group and the state over access to the presumed shipwreck.

When the excavation finally got underway last Friday, divers expected to find an object similar to the Griffin’s reputed size a couple of feet below the surface, based on sonar readings. It’s now believed to be perhaps 10 times farther down. Libert, who says he has spent more than $1 million on his long quest and put the excavation’s price tag at “six figures,” scrambled to obtain equipment that can dig deeper and is better able to break through the hard-packed mud.

It probably will take another day or two to widen the hole and reach the hard surface, Vrana said. The excavation permits expire Friday, although the group could seek extensions. But with the French team scheduled to leave by then, the divers were working faster in hopes of confirming at least the presence of a shipwreck.

State officials and Libert’s group agree if the Griffin is found, it will belong to France because it was operating under authority of King Louis XIV. Graham Paul, a French consul general based in Chicago, visited the team over the weekend and said his government would favor attempting to recover the vessel.

“It would be a major excavation and very costly,” Vrana said.

But the wreckage could be in surprisingly good condition after being encased in cold mud for 334 years because it wouldn’t have been exposed to oxygen, which causes wood and metals to deteriorate, said Dave Miller, an archaeologist with Great Lakes Exploration Group.

“That’s the best way of conservation for all the artifacts and for the hull,” L’Hour said. “One can’t imagine something better than this kind of clay and mud.”