Lake levels of the day were some 250 feet lower, exposing a narrow bridge of land running from one side of Huron to the other. Prehistoric people evidently thought this isthmus was a perfect place to intercept caribou on their seasonal migrations. The hunting site they built, now inundated, opens a window onto prehistoric America and provides valuable evidence in a region where such artifacts are practically non-existent.

If the hunting structures “were on solid ground, (they) probably would’ve been bulldozed away for a Walmart parking lot by now,” says archaeologist Alan Osborn of the University of Nebraska-Omaha and the University of Nebraska State Museum, who was not part of the discovery team. Underwater archaeology is expensive, but “in this case, it’s revealing a site that’s in pretty much pristine condition.”

Serendipity, the researcher’s friend, is to thank for this discovery as well. A half-dozen years ago, the federal government published new maps showing Lake Huron’s underwater ridge, which runs from northeastern Michigan to southern Ontario, as archaeologist John O’Shea was reading a book about Siberian reindeer herders, who laid down brush to direct their animals’ path. O’Shea, of the University of Michigan’s Museum of Anthropological Archaeology, and his colleagues decided to take a long shot and look for similar features on Huron’s underwater ridge.

With the help of sonar, a remote-controlled underwater vehicle and scuba divers, O’Shea’s team eventually found a complicated system of submerged structures at a point where the caribou’s spring and fall migration paths would’ve crossed. In the fall, caribou heading south along the land bridge would’ve made their way straight into a simple cluster of stone hunting blinds.

But animals heading north in the spring marched into a much more systematic form of ambush. The site’s architects carefully placed two parallel lines of boulders to outline a path 26 feet wide and 100 feet long. Caribou naturally follow lines, O’Shea says, so they would’ve walked along this “drive lane” only to hit a dead end created by a natural stone wall. Meanwhile hunters could hide in another clutch of stone hunting blinds built along the lane. The ground here was littered with debris from the manufacture or repair of stone tools, probably spear points, the researchers say in this week’s issue of the Proceedings of the National Academy of Sciences.

The site speaks to the seasonal pattern of the earliest Americans’ lives, O’Shea says. People probably didn’t live on the isthmus. But in the spring, numerous families would’ve congregated at the drive lane, which required perhaps 15 or 16 hunters to operate.

“That doesn’t sound like a huge number, but if these people are living in small family groups most of the year, that’s a pretty significant aggregation,” O’Shea says. People would have socialized as well as hunted before dispersing, he says. Smaller groups would’ve gathered to use the fall hunting blinds. Other prehistoric sites — though none in the Great Lakes — boast similar innovations.

The researchers make a “compelling case,” says Leland Bement of the Oklahoma Archaeological Survey, who was not affiliated with the study team. The site, he says via e-mail, “provides another example of the skill and level of organization of big-game hunters in North America … and the ability of the hunters to plan and execute strategies to intercept these animals.” The find also shows that it’s possible to gain valuable results from underwater exploration, he says.

Such experience may come in handy as researchers try to chart the paths of the first Americans. It’s likely that archaeological sites from the time are submerged, and O’Shea says the new discovery shows the value of underwater searches.

“In the Great Lakes, there was no evidence of what (early Americans) were doing at all,” he says. “By looking in the right place we were able to find them.”