The Great Lakes system is the largest freshwater system on Earth, holding about 21 percent of the planet’s surface fresh water. The five lakes — Superior, Michigan, Huron, Erie, and Ontario — span a combined surface area of roughly 94,000 square miles and drain northeast through the St. Lawrence River into the Atlantic. Their combined shoreline, including islands, runs about 10,000 miles.

Michigan alone has 3,288 miles of Great Lakes shoreline, more than any other state in the contiguous United States. The northern reaches of the lakes sit at roughly the same latitude as Paris, France, while the southern edge aligns with Barcelona, Spain. Scattered throughout the system are about 35,000 islands.

Lake Superior is the largest of the five. Its surface area of 31,700 square miles is larger than Austria. It holds about half of all the water in the Great Lakes system — more than the other four lakes combined. A single drop of water entering Lake Superior takes about 191 years to cycle through the system and reach the Atlantic. Lake Erie, by contrast, flushes its entire volume in about 2.6 years.

About 99 percent of the water in the Great Lakes is ancient fossil water trapped by melting glaciers over 10,000 years ago. Only about 1 percent is replenished each year through rain and runoff. The lakes hold roughly 84 percent of North America’s surface fresh water.

The current international border through the lakes traces back to a map drawn in 1755 by British cartographer John Mitchell. Mitchell had never surveyed most of the territory he mapped. He relied on second-hand accounts, French colonial maps of questionable accuracy, and educated guesses. His map contained significant errors.

In 1783, British and American negotiators used that map to draw the US–Canada border at the end of the Revolutionary War. The border placed certain islands and waterways on the wrong side of the line. Some disputes stemming from Mitchell’s errors were not resolved until the 20th century, and certain legal questions about boundary segments remained open into the 1980s.

Lake Michigan is the only Great Lake located entirely within the United States. The other four are shared with Canada.

There are documented geological and archaeological features in the lakes. In Lake Michigan, researchers found a circular arrangement of stones on the lake floor that resembles Stonehenge. One of the stones has a carving of a mastodon, an animal that went extinct about 10,000 years ago. The site has not been featured in major natural history exhibits.

About 40 meters beneath the surface of Lake Huron, researchers found a submerged ridge with a roughly 9,000-year-old caribou hunting site. It includes stone structures, drive lanes, and blinds built by ancient hunters. The site was preserved when glaciers melted and water levels rose.

Lake Ontario contains a perfectly circular depression on its floor resembling a meteor impact crater. Scientists have studied it and taken core samples, but its origin is officially unconfirmed.

Beneath Lake Huron lies the world’s largest underground salt mine. The Goderich mine extends about 1,800 feet below the lake floor and has operated continuously since 1959.

Over 6,000 shipwrecks lie on the floors of the Great Lakes. Fresh water preserves them better than salt water. Some wrecks more than 100 years old still have visible cargo, intact cabins, and legible name plates.

The lakes produce their own weather effects. In winter, Arctic air passing over relatively warm lake water generates thick columns of steam rising from the surface, a phenomenon known as sea smoke. Waves crashing under hardened ice shelves along shorelines can also cause cone-shaped mounds of ice to erupt, blasting water and slush into the air like geysers. These formations, sometimes called ice volcanoes, have no equivalent on other freshwater bodies on Earth.

The lakes can also generate meteotsunamis, waves triggered by sudden changes in atmospheric pressure rather than earthquakes. A fast-moving storm front can drop pressure so rapidly that the lake surface deforms and a wave propagates outward. These waves can reach several feet in height and can arrive on days that otherwise look calm.

A seiche — a standing wave that bounces from shore to shore — occurred on Lake Erie in 2008 after a powerful windstorm pushed water toward the eastern shore and the wind then stopped. The resulting waves reached 12 to 16 feet and flooded parts of Buffalo, New York.

Lake effect snow is a regular winter feature of the region. In November 2014, a single lake effect storm buried parts of western New York under 77 inches of snow in five days.

The lake floors also contain historical mysteries. The Lake Michigan Triangle, the area between Ludington, Benton Harbor, and Manitowoc, has a documented history of ship, aircraft, and person disappearances over more than a century. Investigators have not produced an explanation for the losses.

There have also been reports since the 1800s of a serpentine creature in Lake Erie, sometimes called Bessie. Sightings have come from ship captains, commercial fishermen, and beachgoers across two centuries. No one has ever definitively explained the reports.

Saturn’s moon Titan has a hydrocarbon lake named Ontario Lacus, named after Lake Ontario because the two are nearly identical in shape and size. One holds fresh water; the other holds liquid methane at about −290 degrees Fahrenheit.

If the water from the Great Lakes were spread evenly across the entire contiguous United States, the whole country would be covered in about 9.5 feet of water.

The Great Lakes system remains defined in many respects by the 1755 Mitchell map. The largest freshwater system on Earth, the source of drinking water for about 40 million people, is still governed by the legal interpretation of a map drawn by a man who never stood on its shores. Disputes directly stemming from the map’s errors were not fully resolved until 1925, and some boundary questions remained open into the 1980s.