From lake monsters and towering sand dunes to shipwrecks and enormous waves, these freshwater beauties are full of surprises.

By Lauren Phillips
April 18, 2017

1. They’re HUGE—6,000,000,000,000,000 gallons huge.

Photo: Goodygreen/Getty Images
Photo: Goodygreen/Getty Images

What's so great about the Great Lakes? Well, their size, actually, may be the most obvious (and accurate) aspect of their greatness. They're huge.

What does 6,000,000,000,000,000 (6 quadrillion) gallons of water look like? It's enough to cover the contiguous United States in almost 10 feet of water. That's a lot of water. 

The Great Lakes are home to almost 20 percent of the world’s freshwater supply (and 84 percent of North America’s). Their total surface area is almost 95,000 square miles—bigger than the combined area of the United Kingdom. If the Great Lakes were a state, they’d be the 11th largest. Lake Superior is the second largest lake in the world, bigger than all but the Caspian Sea. Lake Huron is the fourth largest, and Lake Michigan is the fifth.  

The Great Lakes are so large that their combined shoreline is almost half as big as the circumference of the planet Earth. It’s a 6,500-mile drive around all the lakes. This official Circle Tour would take about 108 hours, or four and a half days, if you drove without stopping.

RELATED: These Spectacular Photos of Waves on Lake Erie Will Blow You Away

2. They are literally inland seas.

Photo: ehrlif/Getty Images
Photo: ehrlif/Getty Images

They may not be coastal in the traditional sense (or have saltwater), but the Great Lakes have many of the behaviors of seas: rolling waves, tidal shifts, strong currents, and great depths, to name a few. Though the tides are small (less than five centimeters at their largest), they do occur twice a day as the gravitational forces of the sun and moon shift. A casual observer probably wouldn’t notice these tides, as they’re almost always hidden by bigger fluctuations in lake levels caused by wind and barometric changes. These include 25-foot-tall waves and massive, dangerous shifts in water levels called seiches or standing waves.

A seiche occurs when wind and atmospheric changes push water from one end of a body of water—in this case, a very large lake—to the other. The water bounces to the opposite side of the area and can continue to move back and forth for hours or even days, sometimes overflowing and causing serious damage.

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The Great Lakes are all connected, with a perpetually flowing current that cycles water through channels that link them. This flowing inland sea cycles water from Lake Superior into Lake Michigan and Lake Huron (which are technically one enormous body of water connected by the Straits of Mackinac). From there, water moves into Lake Erie, and then down Niagara Falls into Lake Ontario. From there, some of it flows down the St. Lawrence River into the Atlantic Ocean. A single drop of water could spend centuries circling in these lakes: It takes 200 years to move through Lake Superior alone.

3. More than 40 million people (not to mention fish, plants, and animals) rely on them.

Photo: ehrlif/Getty Images
Photo: ehrlif/Getty Images

You read that right: More than 40 million people—more than the population of California, the most populous state in the U.S.—get their drinking water from the Great Lakes. More than 3,500 species of plants and animals also live in the Great Lakes basin.

As one of the largest freshwater fisheries in the world, the Great Lakes is home to more than 170 species of fish, including lake sturgeon, which can weigh more than 200 pounds. An estimated 100 million lake trout—nearly 20 percent of the human population of North America—live in Lake Superior alone. Millions of migrating birds also rely on these natural wonders, particularly shallower and warmer Lake Erie, every year.

RELATED: Great Lakes Seafood Dives

4. They’re 10,000 years young.

Photo: csterken/Getty Images
Photo: csterken/Getty Images

The Great Lakes were formed at the end of the last Ice Age, as the glaciers that covered the earth melted about 14,000 years ago. As the glacier covering today’s Great Lakes region melted and retreated north, it left deep depressions in the ground. These depressions filled with water and began to resemble the lakes we know and love about 10,000 years ago, making them babies in geological terms.

For comparison, the Rocky Mountains were formed between 55 and 80 million years ago; the Grand Canyon began its formation 5 to 6 million years ago, and the Ancient Egyptians were building the pyramids about 4,500 years ago. The Great Lakes were formed about the same time that the woolly mammoths were dying out. The dunes that surround the lakes are even younger, formed about 3,000 to 4,000 years ago (more on those below).

RELATED: The 10 Best Things to Do in Grand Haven, Michigan

5. They are surrounded by the largest freshwater coastal dune system in the world.

Photo: Danita Delimont/Getty Images
Photo: Danita Delimont/Getty Images

Sand dunes line all the Great Lakes in both the U.S. and Canada, with some 275,000 acres of dunes around Lake Michigan alone. The enormous system is visible from space and is home to many endangered and threatened animals.

On Lake Superior, the Grand Sable Dunes of Pictured Rocks National Lakeshore rise more than 400 feet above the lake and have 3,200 acres of rolling sand. In Michigan, Silver Lake Sand Dunes has a 2,000-acre sand mountain, and the bluffs in Sleeping Bear Dunes National Lakeshore tower 450 feet above the waters of Lake Michigan. Sleeping Bear Dunes’ Dune Climb is a visitor favorite: Hike to the peak of these enormous dunes, then roll or run back down to the bottom.

6. They’re full of mystery.

Photo: Michigannut/Getty Images
Photo: Michigannut/Getty Images

More than 30,000 people have lost their lives in the more than 6,000 ships that have sunk in these lakes. Hundreds of ships have been lost in a particularly dangerous stretch of water on Lake Superior, earning the area nicknames such as the Graveyard of the Great Lakes and Shipwreck Coast. There’s an area in Lake Michigan known for its strange disappearances and alleged UFO sightings—the Bermuda Triangle of the Great Lakes, perhaps. And everyone knows of the wreck of the Edmund Fitzgerald on Lake Superior. There’s even a Great Lakes Shipwreck Museum to commemorate these tragedies.

And there are plenty of mysteries swimming the waters of the Great Lakes: Lake Erie has a resident monster, reportedly a huge, serpent-like creature known as Bessie, who has allegedly been sighted several times over the years. Lake Michigan had a bit of a pirate problem in its past, with three notorious marauders causing chaos across its waters. The area also has a history of ghost ship sightings.

Photos: Goodygreen/Getty Images; ehrlif/Getty Images (2); csterken/Getty Images; Danita Delimont/Getty Images; Michigannut/Getty Images