Folks living along the shores of Lake Michigan always anticipate the end of winter. After the ice melts, some have a window into another world—as frigid springtime waters reveal thousands of ghostly shipwrecks.

For hundreds of years, cargo ships have sailed Michigan’s Great Lakes. They carried coal, grain, salt, steel, stone, and wood along several important shipping routes. During Michigan’s lumbering heyday, the passage between the North and South Manitou Islands in Lake Michigan boomed. The islands offered shelter. But their sandy shallows could be treacherous during violent storms.

“A time for everything,” says the Preacher of Ecclesiastes. Turns out, Solomon’s observation is true of shipwrecks too. Michigan historians estimate that most Great Lake wrecks—some 6,000 of them—happened during October or November.

The U.S. Coast Guard still patrols the shipping lanes. Lieutenant Commander Charlie Wilson says crews often see submerged ships while on helicopter patrol. The water is so clear that visibility from the air is remarkable. This April, Vintage News, a popular history website, reported on the photographing of several historic wrecks in the Manitou Passage.

Of the five wrecks photographed by the Coast Guard team, only two are known shipwrecks. One is the James McBride. Researchers believe it was the first ship to navigate the entire passage from the Great Lakes to the Atlantic Ocean. This 121-foot wooden ship became stranded on sand on October 19, 1857.

The second photographed wreck is the Rising Sun. It was a 133-foot passenger steamer that ran aground October 29, 1917, as the result of a fall snowstorm. Caught on the rocks near shore, the ship listed to one side and filled with water. According to some accounts, everyone escaped except for one crewman . . . who slept through the entire incident in an unsubmerged part of the vessel. He was rescued the next day.

Scientists say extremely cold water is one reason sunken ships last longer in the Great Lakes. That’s because rust is a reaction of metal atoms with their environment. Higher temperatures make ions react—and therefore rust—faster. Cold water also keeps certain kinds of algae and bacteria at bay.

Another preservation factor in the Great Lakes is fresh water. Salt water speeds up rust formation because electrons react more easily than in fresh.

With a trove of historic shipwrecks, Michigan created an underwater preserve system in the 1980s. Divers who don’t move or remove underwater objects are welcome to explore the wrecks. And with thousands out there, that could take quite a while.