You’ve seen them on a map. Those five blue blobs sitting right in the middle of North America, looking like a giant took a bite out of the border between the U.S. and Canada. Most people just think of them as big lakes. But honestly, that’s like calling the Grand Canyon a nice ditch. When you stand on the shore of Lake Superior in the middle of a November gale, you aren't looking at a pond. You’re looking at an inland sea with waves that can swallow 700-foot ore boats whole.
So, what are Great Lakes in the eyes of a geologist or a local? They are a massive, interconnected freshwater system containing about 21% of the world's surface fresh water. That is a staggering amount of liquid. If you took all that water and spread it evenly across the lower 48 states, the entire country would be under nearly 10 feet of water.
It’s easy to get lost in the numbers, but the vibe is what matters. These lakes have their own weather systems. They have tides—sort of—and they definitely have shipwrecks. Thousands of them.
The five sisters: Not all lakes are created equal
People often lump them together, but each lake has a personality that’s wildly different from the others. You can't just visit one and say you've seen them all.
Lake Superior is the boss. It’s the largest freshwater lake in the world by surface area. It is cold, deep, and deeply intimidating. The water is so cold that it famously "never gives up her dead," a line immortalized by Gordon Lightfoot in his song about the SS Edmund Fitzgerald. Because the water is so chilly, bacteria don't grow as quickly, meaning bodies from shipwrecks often don't resurface. It’s a haunting, beautiful place with rugged granite cliffs and water so clear you can see 30 feet down on a calm day.
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Then you have Lake Michigan. It’s the only one entirely within the United States. It’s got the dunes. If you go to Sleeping Bear Dunes, you’ll find sand towers 450 feet above the water. It feels like the ocean, especially with the Chicago skyline looming over the southern tip.
Lake Huron is the one people forget, which is a shame. It has the longest shoreline of any of the lakes if you count all its islands. It’s home to Manitoulin Island, which is actually the largest freshwater island in the world. It’s a labyrinth of bays and channels that sailors have been navigating—and crashing in—for centuries.
Lake Erie is the shallowest and, frankly, the most temperamental. Because it’s shallow, it warms up fast in the summer, which makes it great for swimming but also prone to massive algae blooms. It also kicks up incredibly dangerous "short" waves during storms because the water has nowhere to go but up.
Finally, Lake Ontario. It’s the smallest in surface area but deeper than Erie. It sits at the base of the Niagara Escarpment. All the water from the other four lakes eventually drains through Erie, over Niagara Falls, and into Ontario before heading out the St. Lawrence River to the Atlantic.
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Why the "Inland Sea" label actually matters
When asking what are Great Lakes, you have to look at the physics. These aren't just big bodies of water; they are "seas" in every functional sense. They create their own "lake effect" snow. If you live in Buffalo or Grand Rapids, you know exactly what this means. Cold air blows over the relatively warm water, picks up moisture, and dumps six feet of snow on your driveway in a single afternoon.
The Great Lakes also have "seiches." It’s not a true tide caused by the moon, but a standing wave caused by wind or air pressure. Think of it like a giant bathtub. If the wind blows hard enough from the west, the water piles up on the eastern shore of Lake Erie, raising the water level by several feet in Buffalo while lowering it in Toledo. It’s weird, and it can be deadly for unsuspecting boaters.
A history carved in ice and commerce
About 10,000 years ago, a massive ice sheet—the Laurentide Ice Sheet—started retreating. It was miles thick. As it moved, it acted like a giant sandpaper block, carving out deep basins in the earth. When the ice melted, it filled those holes. That’s the origin story.
But the human history is just as heavy. For the Anishinaabe and other Indigenous peoples, these lakes (specifically Gichi-gami or Superior) were the center of the world. Later, they became the highway for the industrial revolution. Without the Great Lakes, cities like Detroit, Cleveland, and Chicago wouldn't exist as we know them. They were the path for iron ore, coal, and grain.
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Even today, the "Lakers"—those massive cargo ships—carry millions of tons of cargo. If you’ve ever stood under the Aerial Lift Bridge in Duluth and watched a 1,000-foot freighter slide past, you realize the scale of the economy that still relies on these waters.
The dark side of the water
It isn't all sunsets and sailboats. The Great Lakes are under a lot of pressure. Invasive species are a nightmare here. Zebra and quagga mussels have filtered the water so much that it's clearer than it used to be, which sounds nice, but it's actually bad. They’ve stripped out the nutrients that native fish need.
Then there are the "forever chemicals" like PFAS and the legacy of industrial pollution. We treat these lakes like an infinite resource, but they are a closed loop. Only about 1% of the water in the lakes is "renewed" each year by rainfall and runoff. The rest is ancient meltwater. If we pollute it, it stays polluted for a long, long time.
How to actually experience them
If you're planning to see what are Great Lakes for yourself, don't just go to a city pier.
- Pictured Rocks National Lakeshore: Head to the Upper Peninsula of Michigan. The sandstone cliffs are streaked with colors from minerals—reds, greens, and oranges—towering over the turquoise water of Lake Superior.
- The Bruce Peninsula: In Ontario, Canada, this spot offers Caribbean-looking water surrounded by ancient cedars. It’s rugged and spectacular.
- Mackinac Island: No cars allowed. You take a ferry over Lake Huron to an island that feels frozen in the 1890s.
- Door County: Wisconsin’s "thumb" into Lake Michigan. It’s all about cherry orchards, lighthouses, and fish boils.
Practical steps for your first Great Lakes trip
Stop thinking of this as a "lake trip" and start thinking of it as a coastal expedition. The distances are huge. Driving around Lake Superior (the Circle Tour) takes about 1,300 miles.
- Check the marine forecast, not the local news. If you’re getting on a boat, the weather in town is irrelevant. You need to know the wave heights. A 4-foot "chop" on the Great Lakes is much more violent than a 4-foot swell in the ocean because the period between waves is shorter.
- Pack layers even in July. The "lake breeze" can drop the temperature by 20 degrees in minutes. I’ve seen people shivering in shorts in Marquette when the wind shifts off the water.
- Respect the cold. Even in summer, Lake Superior stays around 40-50 degrees Fahrenheit in its deep spots. Hypothermia is a real risk if you fall in, even on a sunny day.
- Download a ship tracking app. Use something like MarineTraffic to see what massive freighters are nearby. Seeing a 1,000-footer pass by is a core Great Lakes experience.
These lakes are a global treasure, a geological fluke, and a dangerous playground all at once. They define the heart of a continent. Whether you call them the "Third Coast" or the "Sweetwater Seas," they are something you have to feel to understand. Keep the water clean, watch the horizon for storms, and never underestimate the power of a "lake."