The water reared up and slammed onto the sand like an ambush predator. Then it withdrew, and came back. Again and again the surf attacked the beach, and exploded over a nearby concrete breakwater. White gulls stood out against a dull sky as they fought to stay aloft in winds that had gathered strength over hundreds of miles of open water. The National Weather Service had issued a gale warning, an alert used only for marine locations. But this wind advisory wasn’t for the coast along Cape Cod or California, and the spray from the angry surf wasn’t salty. This stormy seaside scene unfolded on a spring day along the shores of a mere lake, far from the nearest ocean.

The Great Lakes of North America’s midsection—Superior, Michigan, Huron, Erie, and Ontario—together span nearly 100,000 square miles, with a combined coastline just shy of 10,000 miles. They hold more than a fifth of Earth’s unfrozen fresh water, straddle an international border, and help move more than $15 billion dollars worth of cargo each year. They even have their own U.S. Coast Guard district, the only lakes with such a distinction. And the Guard’s rescue teams stay busy: Superior and its siblings are capable of storm surges, rip currents, tsunamis, rogue waves, unique extreme weather phenomena, and destructive surf. They have claimed more than 6,000 ships, more than the Gulf of Mexico and the Black Sea combined, according to estimates. So should we really be calling them the Great Inland Seas?

“The most accurate answer you’re going to get is, ‘I don’t know,’” says John Richard Saylor, author of the upcoming Lakes: Their Birth, Life, and Death. “I do think it comes down to semantics, what you want to call a ‘sea.’”

For many, the Great Lakes are indeed greater than lakes. The United States Environmental Protection Agency, for example, describes them as “vast inland freshwater seas.” A seminal 2017 paper in Limnology and Oceanography, authored by some of the most influential researchers studying the lakes, also refers to them as ‘inland seas.’ But what makes a sea varies by source.

Superior, Michigan, Huron, Erie, and Ontario (left to right), hold more than a fifth of the planet's fresh water.
Superior, Michigan, Huron, Erie, and Ontario (left to right), hold more than a fifth of the planet’s fresh water. NASA Earth Observatory/Public Domain

You might, for example, associate seas with saltwater, but “Whether water is salty or fresh does not cleanly separate lakes from seas,” says Robert Sterner, director of the Large Lakes Observatory at the University of Minnesota Duluth and one of the coauthors of the 2017 paper. “Many lakes are saline, some more so than the oceans. And the Baltic Sea is pretty fresh, especially near its headwaters.”

Generally speaking, seas are defined by size and borders—but, from there, things get murky. The U.S. National Oceanic and Atmospheric Administration (NOAA), for example, describes seas as smaller than oceans and at least partially bounded by land (the Sargasso Sea, in the center of the Atlantic Ocean and defined by surrounding currents, is an exception). Merriam-Webster has more than half a dozen definitions for the word “sea,” including “an inland body of water,” citing the Caspian Sea as an example. However, most scientists actually consider the Caspian a lake because it is surrounded by land with no connection to an ocean. “I generally say the Caspian Sea is not on the list of Earth’s large lakes mainly for cultural, not scientific, reasons,” says Sterner.

Let’s muddy the waters a bit more: The Dead Sea is a saltwater lake, and the Gulf of Mexico and Hudson Bay are both seas. Meanwhile, a growing number of scientists believe the Red Sea may actually be a young ocean.

<em>Typo</em>, which sank in northern Lake Huron in 1899, is a popular wreck in the Thunder Bay National Marine Sanctuary, and one of more than 6,000 ships claimed by the Great Lakes.
Typo, which sank in northern Lake Huron in 1899, is a popular wreck in the Thunder Bay National Marine Sanctuary, and one of more than 6,000 ships claimed by the Great Lakes. NOAA’s National Ocean Service, CC BY 2.0

Back in the Midwest, even the people who have known the Great Lakes the longest haven’t settled on exactly what they are. The lakes themselves began emerging more than 10,000 years ago when mile-high glaciers, which had carved basins and gashes into the land, retreated at the end of the last ice age. Meltwater filled depressions left behind, creating first marshy areas and then deeper bodies of water over several millennia—and humans were there to witness it. A 2021 paper in the journal PaleoAmerica described initial research at Belson, a site in Michigan that may be 13,000 years old and represents some of the earliest evidence of humans in the Great Lakes region. Also in 2021, in the journal PLOS One, another team of researchers described 9,000-year-old flakes of obsidian, evidence of toolmaking, found at the bottom of Lake Huron. Interestingly, the material’s chemical composition indicates that the volcanic glass originated in Central Oregon, about 2,500 miles to the west, hinting at extensive travel and trade networks across the continent at the time.

While the languages of these earlier people are not known, over the last few millennia the Great Lakes have been home to several Native American and First Nations peoples, most of which belong, culturally and linguistically, to the Anishinaabe. The term covers a number of communities dispersed over a broad and varied geographical area, but there is continuity among them in how the Great Lakes are perceived.

“I refer to Anishinaabe as a confederacy of different nations, which include Ojibwe, Potawatomi, and Odawa, but also Menominee, Saulteaux, Meskwaki, Kickapoo, Shawnee, Waabanaki—all of these tribes speak various dialects of the same language,” says Michael Waasegiizhig Price, a Wisconsin-based traditional ecological knowledge specialist for the Great Lakes Indian Fish & Wildlife Commission. Price notes that there are different dialects across the region, and varied orthographies—the ways a spoken language is written down, including spelling and capitalization. Each lake also has a specific name but, when talking generally about one of the Great Lakes, Anishinaabe people say gichigami, gichi-gami, or chigamiing, depending on dialect, which all translate as “big or great body of water.” Whether that means “lake” or “sea,” well … The Decolonial Atlas, which Price cites, refers to the Great Lakes as “the five freshwater seas,” but he takes exception to that.

“As for the word ‘sea,’ I believe this to be a colonial word that was brought here (by) Europeans,” he says. “Today, some Anishinaabe people refer to the Great Lakes as inland seas, but this is a new evolution in our language—English is impacting how we use our Indigenous language.”

So, the semantics remain imprecise across languages, but what about the science? Do the people studying the Great Lakes see them as inland seas? The answer is a resounding “sort of.”

Sterner of the Large Lakes Observatory, like many other scientists who study the lakes, calls them the “Laurentian Great Lakes.” It’s a nod to their birth from the remnants of the Laurentide Ice Sheet, and distinguishes them from East Africa’s Great Lakes, in the Rift Valley region, where plate tectonics are slowly pulling the continent apart. The Midwestern lakes, he notes, are closer to seas in some aspects. “Earth’s rotation is an important force in steering currents in large lakes but not small ones. That’s a characteristic large lakes share with oceans and seas, but I don’t know if you’d say that makes them a sea,” says Sterner, adding, “I think not.”

Scientists might not classify the big lakes as seas, but they often study them as if they were.

“The modelers here who work on things like currents and waves and ice use ocean models,” says Lauren Fry, a principal investigator at NOAA’s Great Lakes Environmental Research Laboratory (GLERL) in Ann Arbor, Michigan. In fact, fieldwork on the Great Lakes is typically carried out using equipment designed for oceanography, including remotely operated vehicles, deepwater probes called sondes, and repurposed marine watercraft, such as R/V Neeskay, a steel-hulled former Army T-boat that conducts year-round research on Lake Michigan for the University of Wisconsin-Milwaukee’s School of Freshwater Sciences.

In Lake Huron's Saginaw Bay, a NOAA team deploys buoys that will help provide real-time coastal monitoring, similar to oceanic observation.
In Lake Huron’s Saginaw Bay, a NOAA team deploys buoys that will help provide real-time coastal monitoring, similar to oceanic observation. NOAA GLERL

For some who study the Great Lakes, relative size is a defining factor, particularly with Lake Superior, which is the world’s largest lake by surface area (aside from the much larger Caspian Sea—if you’re willing to call it a lake). “Compared to other bodies of water in the U.S., I would definitely classify it as an inland sea,” says Teso Coker, a water resource scientist at the University of Minnesota Duluth. “When I’ve shown videos and pictures to my colleagues overseas of Lake Superior, they have all said, ‘That is a sea!’”

Fry, however, would not call the Great Lakes inland seas—at least not scientifically. Despite their size, the lakes are beholden to what happens on the land that surrounds them in a way larger seas are not. For example, precipitation and runoff that drains into the lakes significantly affects their water levels, chemical composition, and other characteristics.

“As a hydrologist, I have to say that, because of the influence of the land surface processes on the lakes, I think it’s tough to call it an inland sea,” says Fry. She does, like most scientists studying the lakes, refer to them as a single system. “I’d call it a system of large lakes, connected by channels.”

But, adds Fry, “I would fall into different camps depending on whether I’m wearing my hydrologist’s hat or my canoer hat. I’m a paddler and, in terms of safety, you have to think about it as if you’re on a sea.… It’s really big water. We have rip currents. We have storm surges. We have meteotsunamis.”

Yes, tsunamis have been documented on all five of the Great Lakes—at least, meteotsunamis, which have the same characteristics as tsunamis created by seismic events such as earthquakes and landslides. As their name suggests, meteotsunamis are generated by severe storms and other extreme meteorological events. They travel across large areas of water, growing in height, or amplifying, over shallows before crashing ashore. Meteotsunamis tend to be smaller and less destructive than seismic tsunamis, but can still be dangerous: One such wave-gone-wrong in Lake Michigan in June 1954 killed seven people in Chicago.

Studying meteotsunamis is a relatively new area for Great Lakes science: For many years, the phenomenon was grouped with seiche events, tide-like conditions that also can occur in all five lakes. Seiches are basically the sloshing back and forth of water across the entire lake over several hours or even days.

Seiches, says Fry, are particularly common on Lake Erie, which is aligned southwest to northeast, just like the path of most of the region’s weather systems. “It starts with a storm surge just piling up on one end,” Fry says. “You could be in Buffalo and the water would be several feet higher than in Toledo.” Eventually, that water sloshes toward Toledo and then back again, its energy dissipating over time.

Lake Superior is notorious for dangerous waves of a different kind: clusters of rogue waves. These abnormally large waves are colloquially known as “the three sisters” because they appear to travel as a trio, the second and third wave swamping a ship before it recovers from the first battering. While scientists are still trying to understand how the waves form, the phenomenon has been implicated in the tragic 1975 loss of Edmund Fitzgerald; shortly before the ship went down, the captain of another vessel nearby reported being hit by multiple 30- to 35-foot waves in quick succession; the monster waves were heading in the direction of the doomed freighter.

<em>Edmund Fitzgerald</em> in 1971, four years before it sank in Lake Superior during a storm, possibly overwhelmed by the unpredictable lake's "Three Sisters" rogue waves phenomenon. The entire crew of 29 was lost.
Edmund Fitzgerald in 1971, four years before it sank in Lake Superior during a storm, possibly overwhelmed by the unpredictable lake’s “Three Sisters” rogue waves phenomenon. The entire crew of 29 was lost. Greenmars, CC BY-SA 3.0/Wikimedia

In fact, the strongest case for describing the Great Lakes as inland seas may be to remind the public of the potential threat that they pose. About 100 people drown each year in the Great Lakes.

“When folks come to Lake Superior to go kayaking or boating, it’s a much different experience than on a small lake,” says Natalie Chin, climate and tourism outreach specialist for Wisconsin Sea Grant. “They can be very dangerous. Respecting the power of the Great Lakes is very important.”

Chin is based in Superior, a city at that lake’s southwestern tip. (The very name of her organization hints at the special status of the Great Lakes: Sea Grant is a national network of universities and federal and state partners, mostly along the Atlantic and Pacific, that are studying and conserving coastal resources.) She’s currently developing a kayaking safety campaign for Lake Superior and, while she doesn’t think a formal name change is in order, she sees the value in emphasizing its sea-like qualities.

“It comes down to the question of how do we keep people safe, how do we convey that these lakes are unpredictable and there can be very dangerous situations that occur,” says Chin.

Perception of the Great Lakes is, in the end, in the eye of the beholder. Modeled beside the Gulf of Mexico or the Black Sea, even mighty Lake Superior seems small. But when you’re tossed around in a boat on any of the Great Lakes during a storm, you’d swear you were in the open ocean.

Lake Superior batters a Minnesota shoreline.
Lake Superior batters a Minnesota shoreline. Layne Kennedy/Getty Images

“To me, the distinction between lake and sea is more about language and culture than science,” says Sterner.

“I am attracted to the romance of thinking of large lakes as seas, and referring to them that way certainly makes an important point about large lakes being different in important ways from most lakes on Earth,” he adds. “But my opinion is that there is no unassailable scientific logic for lumping large lakes, say, with the Aegean Sea and not with other, smaller lakes.”

University of Minnesota’s Coker suggests that the Great Lakes belong “in a category of their own.” Perhaps it’s not the lakes that fail to fit our definitions, but rather our words failing to describe their unique nature.

“Chigamiing or gichi-gami are much older words than sea or ocean,” says Price. “In my opinion, there is no difference between a large lake and an inland sea—both are chigamiing.”