"They said, 'Would you be interested?' " recalled Frank Pranschke, the retired electrician who also is an amateur geologist. "I said, 'Hell, no.' They went back down and got me a stump and a videotape. I became very interested."
The result was the discovery of an 8,000-year-old forest, a significant find that has given researchers new insight into the geologic history of the Great Lakes.
"We were staggered to find 8,000-year-old wood in Lake Michigan," said Charles Shabica, head of the Earth sciences department at Northeastern Illinois University, who is coordinating research into the find. "We're really going to have to redo what we know about Lake Michigan."
Pranschke, who since retirement has been pursuing his geology hobby full- time as an unpaid researcher with Shabica, eventually received three of the waterlogged tree stumps, retrieved from beneath 85 feet of water.
The stumps, carbon-dated to determine their age, are the remains of oak and ash trees that grew near what was then the shore of a smaller lake, Shabica said. But the fact of their preservation means the trees must have been inundated by rising lake water before they had a chance to die and decompose, he added.
"For those things to have been preserved that long, they had to have been underwater all that time," Shabica said.
That gives scientists something they haven't had up to now, according to Shabica. "This is the first piece of solid evidence for a lake level at a position in time," he said. "We now know that the lake was quite a bit lower than it is now."
Soft, almost spongy, the stumps sitting in his laboratory refrigerator are cone-shaped, about 2 1/2 feet tall and less than a foot in diameter. Knotholes, rings and even some ancient cracks are visible, offering researchers clues to the kind of climate that existed when they were alive.
The evidence helps fill in what has been a fuzzy period in the geologic history of the Great Lakes, a history that also includes periods when the giant inland seas were larger, the scientists say.
The surface of Lake Michigan, for example, is today about 577 feet above sea level. Four thousand years ago, it was closer to 620 feet, as ancient beaches in what are now Chicago's western suburbs attest.
But 8,000 years ago, the lake, whose southern end is wide enough now to comfortably swallow New Jersey, must have been significantly narrower, scientists say. At that time, the glaciers that formed the lakes had retreated north above Lake Superior, and the melting ice was flowing south, filling up the lakes' modern-day basins.
For Pranschke, the evidence also represents a kind of personal victory. Several years ago, he argued in an academic paper that the lake's low-water point occurred between 8,000 and 9,000 years ago, disputing an earlier theory that put it even earlier. "Now this is confirming what I did," he said.
But the discovery was only the beginning of an extensive period of research for Pranschke and his associates. Since last month, crews from the U.S. Geological Survey have been combing the area where the tree remains were found, using sophisticated sonar to produce detailed underwater maps.
And the amateur salvage divers who first discovered them have been enlisted to return with cameras and measuring devices, in between their trips to hunt for sunken ore carriers and 19th-century steamers. "It's one of those hobbies that, if you really want to do it, becomes all-consuming," said Taras Lyssenko, a member of the diving crew.
Shabica is enthusiastic about the discovery's prospects.
"There's a whole variety of stories that start to come into focus," he said. The successful preservation of tree remains for 8,000 years should encourage scientists to intensify efforts to search the vast lakes.
"If there are wrecks out there, they ought to be in great shape," he said. "If there's an Indian canoe out there that went down 3,000 years ago, it ought to be in great shape."