They're known as America's inland seas.

The North American Great Lakes

Huron,

Ontario,

Michigan,

Erie,

and Superior

are so massive that they border eight states

and contain 23 quadrillion liters of water.

That's enough to cover the land area of the contiguous United States

three meters deep.

These vast bodies of water span forest,

grassland,

and wetland habitats,

supporting a region that's home to over 3,500 species.

But how did such a vast and unique geological feature come to be?

The story begins near the end of the last ice age over 10,000 years ago,

a time when the climate was warming

and the glaciers that cloaked the Earth's surface began their slow retreat.

These immense ice sheets carved out a series of basins.

Those basins filled with water as the ice began to melt,

creating the world's largest area of freshwater lakes.

Over time, channels developed between these basins,

and water began to flow in an ongoing exchange

that persists to this day.

In fact, today, the interconnected Great Lakes

contain almost 20% of the world's supply of fresh surface water.

The water's journey begins in the far north of Lake Superior,

which is the deepest, coldest, and clearest of the lakes,

containing half the system's water.

Lake Superior sinks to depths of 406 meters,

creating a unique and diverse ecosystem that includes more that 80 fish species.

A given drop of water spends on average 200 years in this lake

before flowing into Lake Michigan or Lake Huron.

Linked by the Straits of Mackinac, these two lakes are technically one.

To the west lies Lake Michigan,

the third largest of the lakes by surface area.

Water slowly moves through its cul-de-sac shape

and encounters the world's largest freshwater dunes,

many wildlife species,

and unique fossilized coral.

To the east is Lake Huron, which has the longest shoreline.

It's sparsely populated, but heavily forested,

including 7,000-year-old petrified trees.

Below them, water continues to flow southeastwards

from Lake Huron into Lake Erie.

This lake's status as the warmest and shallowest of the five

has ensured an abundance of animal life, including millions of migrating birds.

Finally, the water reaches its last stop by dramatically plunging

more than 50 meters down the thundering Niagara Falls

into Lake Ontario, the smallest lake by surface area.

>From there, some of this well-traveled water enters the St. Lawrence River,

eventually reaching the Atlantic Ocean.

In addition to being a natural wonder,

the perpetually flowing Great Lakes bring us multiple benefits.

They provide natural water filtration,

flood control,

and nutrients cycling.

By moving water across more than 3,200 kilometers,

the Great Lakes also provide drinking water for upward of 40 million people

and 212 billion liters a day for the industries and farms

that line their banks.

But our dependence on the system is having a range of negative impacts, too.

The Great Lakes coastal habitats are being degraded and increasingly populated,

exposing the once pristine waters

to industrial, urban, and agricultural pollutants.

Because less than 1% of the water leaves the Lake's system annually,

decades-old pollutants still lurk in its waters.

That creates dead zones in some regions where no plants or animals can survive.

Humans have also inadvertently introduced

more than 100 non-native and invasive species into the lakes,

such as zebra and quagga mussels, and sea lampreys

that have decimated some indigenous fish populations.

On a larger scale, climate change is causing the waters to warm,

thus reducing water levels and changing the distribution of aquatic life.

Luckily, in recent years, governments have started to recognize the immense value

of this natural resource.

Partnerships between the United States and Canada are underway to reduce pollution,

protect coastal habitats,

and halt the spread of invasive species.

Protecting something as massive as the Great Lakes system

will require the collaboration of many organizations,

but the effort is critical

if we can preserve the wonder of this flowing inland sea.