updated 12/19/2006 1:11:38 PM ET 2006-12-19T18:11:38

Scientists hope to learn more about what the Great Lakes' shorelines looked like about 10,000 years ago. They explored a limestone land bridge that went from Alpena to Goderich, Ontario — a distance of about 125 miles — and an underwater forest of petrified trees in Lake Huron.

The 2006 research, in which more than 500 dives were made, is the subject of a documentary film, "Great Lakes, Ancient Shores, Sinkholes." It premiered recently at the Cranbrook Institute of Arts in Bloomfield Hills, The Oakland Press reported in a story published Monday.

Another study is planned for 2007 and should result in a second film, "Great Lakes, Ancient Shores," said Luke Clyburn, lieutenant commander of the Great Lakes Division of the U.S. Naval Sea Cadet Corps and a Great Lakes ship captain.

"What we are learning about the Great Lakes of several thousand years ago may change the way we think of this area," Clyburn said.

Clyburn and other scientists have been filming in the Great Lakes for at least 25 years.

There is a petrified forest in 40 feet of water in Lake Huron about two miles offshore from Lexington, he said. Some of the trees have been carbon-dated to indicate they are 6,980 years old.

The Straits of Mackinac, a passage between lakes Michigan and Huron, have been spanned by the Mackinac Bridge since the mid-1950s but didn't exist several thousand years ago, Clyburn said.

"Lake Michigan was much higher than Lake Huron, and the two did not join as they do today at the straits," he said. But water from Lake Michigan seeped underground toward Lake Huron and the two bodies of water eventually became connected.

Clyburn's current film focuses on a sinkhole in Lake Huron about two miles from Alpena near Middle Island. In prehistoric times, the sinkholes were on dry land. Native Americans lived near these sinkholes because they provided water, which attracted game, he said.

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