Scientists and divers have explored a limestone land bridge that went from Alpena to Goderich, Ontario, and an underwater forest of petrified trees in Lake Huron during research this year. They intend to resume exploration in 2007 and hope to learn more about what the Great Lakes' shorelines looked like about 10,000 years ago, at the end of the last Ice Age.

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The 2006 research, in which more than 500 dives were made, is documented in "Great Lakes, Ancient Shores, Sinkholes." It premiered recently at the Cranbrook Institute of Arts in Bloomfield Hills.

The 2007 study is intended to lead to another film, "Great Lakes, Ancient Shores," said Luke Clyburn, 64, of White Lake Township. Clyburn is a lieutenant commander of the Great Lakes Division of the U.S. Naval Sea Cadet Corps and a Great Lakes ship captain.

He alone made 80 dives in 2006, he said.

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

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

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

The age of the trees means the shores of the Great Lakes were radically different from what they are today.

He also said the Great Lakes have "a tremendous number of sinkholes."

In prehistoric times, the sinkholes would have been on dry land. Native Americans lived near these sinkholes, Clyburn said, because they provided water and water attracts game. The existence of the sinkholes could spawn work by anthropologists interested in how these ancient people lived.

The Straits of Mackinac, spanned by the Mackinac Bridge since the mid-1950s, 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 to where Lake Huron and the two bodies of water eventually merged.

Clyburn's current film focuses on a sinkhole that is in Lake Huron about two miles from Alpena near Middle Island.

The project is supported by Michigan Coastal Management Program, the Michigan Department of Environmental Quality, the National Oceanic Atmospheric Administration, the Great Lakes Division of the U.S. Naval Sea Cadet Corps. the Cranbrook Institute of Science, Oakland University and the Noble Odyssey Foundation.