How the Earth Was Made

>> Earth. A 4.5-billion-year-

old planet, still evolving.

As continents shift and clash,

volcanoes erupt, glaciers grow

and recede, the Earth's crust

is carved in numerous and

fascinating ways, leaving a

trail of geological mysteries

behind.

In this episode, the Great

Lakes of North America, the

largest expanse of freshwater

on the planet, are

investigated.

They hold 20 percent of the

world's freshwater and

provide drinking water for

nearly ten percent of

Americans.

These five lakes are among the

world's greatest natural

wonders. But their origins are

a mystery. Now geologists are

investigating, piecing

together the clues that lie

hidden in this extraordinary

landscape, delving deep into a

vast underground salt mine

behind the torrential flow of

Niagara Falls, climbing a

mile-high glacier, where

clues to understanding the

Great Lakes' formation also

provides a window into the

formation of the Earth itself.

S01x07 Great Lakes Original Air Date on March 24, 2009

== sync, corrected by elderman ==

The five Great Lakes,

Superior, Michigan, Huron and

Erie, pour over one of the

world's great waterfalls,

Niagara, into Lake Ontario.

The mighty torrent of the

falls empties excess water

from four of the five Great

Lakes out to the sea.

For geologists, the lakes are

a natural wonder and a puzzle,

and scientists are on the

trail of how they were formed,

with rocks as their clues, and

ice, lava and water as their

suspects.

Their investigation begins at

these seemingly ordinary

industrial buildings beside

Lake Huron.

Hundreds of feet below ground

here, there's a remarkable

secret.

Deep below Lake Huron, and

also Lake Michigan, are vast

salt mines carved out directly

beneath freshwater lakes.

>> Right now we're at 1,750

feet below the surface of the

Earth. We're in the largest

underground salt mine in the

world. And we're below Lake

Huron, a large freshwater

lake.

>> Amazingly, this salt

deposit was uncovered by

accident.

>> They were drilling for oil.

And they hit salt.

And that was the end of

looking for--looking for oil.

They just kept on digging for

the salt.

>> This salt deposit is the

investigator's first clue,

evidence that there was once

an ancient sea here.

>> Many years ago, the--the

salt was formed in a great

salt lake and the evaporation,

dry seasons, the salt dropped

out, evaporated out, and

formed this salt that we're

actually mining in.

>> There are hundreds of

layers of salt, leading

investigators to conclude the

sea must have dried up and

refilled hundreds of times.

Scientists would later prove

this sea finally evaporated

millions of years ago.

35 percent of North America's

salt comes from these mines -

salt used to melt ice on

frozen roads and sidewalks,

salt used to season food - the

remains of million-year-old

seas. All coming from beneath

Lake Huron and Lake Michigan.

>> McCUE: The salt deposit is

massive. There's probably

trillions of tons of salt in

the deposit. It extends all

the way down to Detroit.

All of Lake Huron, the salt is

under it. And all of Michigan.

>> The salt is soft, and over

millions of years the salt

layers should have worn away.

Why haven't they?

It's because the salt is

protected by a vast

impenetrable layer of rock

that lies like a giant basin

beneath Lakes Michigan and

Huron and stretches under Lake

Erie. Like the porcelain

lining a bath tub, the rocky

basin holds the lakes'

freshwater.

Geologist John Zawiskie

and a team of divers are

hunting for clues to the rocky

basin's origins. They're

heading for Thunder Bay, a

small island at the edge of

Lake Huron.

As he walked along

the beach, Zawiskie discovered

some crucial evidence,

seemingly insignificant rocks

that were overlooked for

decades. But Zawiskie suddenly

realized what he was looking

at - fossilized remains of

ancient sea creatures.

>> ZAWISKIE: I was seeing

something that many geologists

had never seen when they

visited this island. There

were the heads of giant

lime-secreting sponges that

were some of the main reef

builders.

>> Zawiskie uncovered a

perfectly preserved fossil of

a giant sea sponge that must

have come from an ancient

coral reef.

For the past five years,

Zawiskie's divers have been

surveying the lake to discover

the size of the ancient coral

reef. They believe it's

hundreds of feet thick and

extends deep below Lake Huron.

And Zawiskie has proof these

rocks are extremely old.

>> The time period can be

pretty confidently bracketed

at right around 385 million

years ago.

>> America was then a very

different place. 385 million

years ago, its land mass lay

in the southern

hemisphere, a land covered by

ancient warm coral seas.

>> This region was just south

of the equator, in tropical

conditions, and shallow seas

had swamped many of the land

areas of the Earth at that

time.

>> Year in, year out, coral

reefs decay naturally and turn

into a soft rock, limestone.

And much of the rock

Zawiskie's divers find under

Thunder Bay Island consists of

layer upon layer of this

limestone from successive

coral reefs. But millions of

years ago, some of this soft

limestone near the surface was

changed. When the salty briny

sea evaporated it turned the

limestone into a second, much

harder rock, something which

would decide the very shape

of the Great Lakes.

>> This rock is limestone.