[narrator] We have uncovered
a remarkable planet,
a world of extreme weather,
alien landscapes,
and bizarre phenomena.
It's the most complex system
we know of in the universe.
That planet...
is Earth,
and we are only beginning
to unlock its secrets.
They are the heartbeat
of the ocean.
But they have a secret.
[man] It would be like
being hit by a car
traveling at 25 miles per hour.
[narrator] Some can rage
out of control.
[man] You've got water
flowing into the subways.
You've got water
at the World Trade Center.
[narrator] They're
called tides...
generated by the celestial pull
of a moon and sun,
aggravated by weather and land,
and wielding
the power to trigger
some of the deadliest disasters
on the planet...
[indistinct shouting]
[narrator]
...on Secrets of the Earth.
Even from space,
it's clear that one feature
separates Earth from all
the rest of the planets
in this solar system--
over 70% of its surface
is covered with water.
Where the sea meets the land,
the waters rise and fall
in predictable patterns
known as tides.
But closer inspection reveals
that sometimes these tides
get out of control.
[man] When a tide is coming in
slowly at the seashore,
it may seem kind of meek,
but tides actually have
tremendous power.
[narrator] What makes the gentle
heartbeat of the oceans
rage in certain places
on the planet?
Here in a 50-mile-long inlet
off the eastern coast
of Canada,
tides are taken
to their most extreme.
This is the Bay of Fundy,
home of the highest tides
on the planet.
A typical high tide
is about two to four feet,
but throughout the year,
the tides in the Bay of Fundy
can rise to over 50 feet,
the height
of a five-story building.
[man] 115 billion tons of water
comes into the Bay of Fundy,
and 115 billion tons of water
goes out of the Bay of Fundy.
To put that in context,
the force of the water
moving in and out
of the Bay of Fundy
is equivalent to
about 8,000 locomotives
pulling that water
in and out of the bay.
[narrator] The sheer load
of water
moving in and out
of the Bay of Fundy
actually tilts the province
of Nova Scotia
back and forth
by one-sixteenth of an inch
during each tide cycle.
If the energy of these tides
could be harnessed,
they could generate
upwards of 10,000 megawatts
of electricity,
enough to power
over three million homes a day.
These powerhouse tides
not only achieve
awesome vertical heights,
they also span almost 200 miles
of coastline.
So when the water retreats,
it exposes
hundreds of thousands of acres
of the ocean floor.
[Pufahl] We're at low tide
at Alma Harbor.
And if this were high tide,
I'd be six feet underwater.
And as the tide
continues to go down,
I'd be able to walk out
for one to two miles.
[narrator] But what is
the secret that drives
these mysterious waters
in the Bay of Fundy?
All tides on Earth,
no matter how extreme,
are powered
by a force from space.
[Filippenko]
Tides are mostly caused
by the gravitational influence
of the moon
and to a lesser extent,
that of the sun.
The moon pulls
on the near side of the Earth
more than it pulls
on the center,
causing the near side
to bulge out
toward the moon
a little bit.
It also pulls on the center
more than on the far side,
causing the far side
to bulge out from the center.
So there are two bulges
on either side.
This bulge consists of water
that's bulging toward the moon
and away from the moon
and to a lesser extent,
toward the sun
and away from the sun.
[narrator] As the moon
revolves around the Earth
and the planet orbits the sun,
these tidal bulges
cause the water
to rise and fall,
producing high tides
and low tides.
The tides in the Bay of Fundy
begin with the gravitational
pull of the moon,
but they are taken
to the extreme
by the bay's unique
funnel shape.
As it juts in from the sea,
the inlet quickly narrows
and becomes extremely shallow.
When the tide rushes in,
the water piles up.
But there's another secret
that propels these tides
to astonishing heights.
In addition to the small waves
you see crashing on the shore,
there is a larger
overall rocking motion
in any body of water.
In the Bay of Fundy,
the time it takes
for this large rocking motion
to go from the mouth of the bay
to shore and back again
is around 12 hours.
Since ocean tides
happen to rise and fall
around every 12 hours, too,
at the precise time
the water has rocked back
toward the mouth of the bay,
another high tide arrives
to push it back in.
This reinforces the rocking,
amplifying the tide upwards
to 50 feet.
[Pufahl] Not two hours ago,
the boats were
about 15 feet higher
at the high tide mark,
the high water mark,
which is that change in color
you can see over there
between dark and light.
The boats, they're now
sitting on the sea floor.
[narrator] For hundreds
of millions of years,
the tides in the Bay of Fundy
have chiseled away
at sandstone rock
to form dramatic
coastal structures,
like these towering sea stacks
that look like
gigantic flowerpots.
[Pufahl] With time,
as coastal erosion continues,
these sea stacks will
essentially collapse,
and new ones will form.
[narrator] The tides
in the Bay of Fundy
not only erode the shoreline,
they also have
the power to kill.
People occasionally
find themselves strolling
a mile from shore
when the tide turns.
January 20, 2012.
Two fishermen drowned when
searching for lobster traps
during low tide
in the Bay of Fundy.
[Pufahl] Tides
in the Bay of Fundy
come in at the rate
of an inch every minute.
So they're incredibly fast.
You could potentially
be swept away
from these strong
tidal currents.
[narrator] The Bay of Fundy
may have
the highest ocean tides
on Earth,
but halfway across the planet,
a different type
of treacherous tide
rules the bay.
There is a place on Earth
where an extreme tide can
sometimes be downright vicious.
The east coast of China.
Here, just south of Shanghai,
the waters
of the South China Sea surge
through the bottleneck-shaped
Hangzhou Bay.
As water piles up
at the narrow mouth
of the Qiantang River,
the incoming tide
forms a long wave of water
that barrels up the river
against its current,
rising to 30 feet tall
and moving
at 25 miles per hour.
It's known as a bore tide.
Unlike a normal tide,
this tidal phenomenon
occurs when a forceful
incoming tide
creates a wave of water
that pushes up
a narrowing river or bay
against the current.
[Pufahl] The standing wave forms
because the water
that's building up
can't dissipate fast enough
because the tide's
rising so quickly.
And in the Qiantang River,
the tidal bore is
consistently 30 feet high.
[narrator] The Qiantang River
bore tide occurs twice a month
when the sun and moon
collectively exert
their gravitational pull
on the Earth.
[Filippenko] Twice per month,
when we have new moon
or full moon,
when the sun, moon,
and Earth are aligned,
the tides raised by the sun
add to the tides
raised by the moon,
so you get
exceptionally high high tides.
These are known as spring tides
because they spring forth.
[narrator] While a full
or new moon
can produce extra-high tides,
there's another
celestial secret
that takes the bore
to the extreme.
On the 18th day
of the eighth lunar month,
over 170,000 tourists
attend the annual
Qiantang River
bore tide festival.
It coincides with
the autumnal equinox,
when the center of the sun
is on the same plane
as the planet's Equator
and the gravitational pull
is at its peak.
That's when the Qiantang River
produces the biggest bore
on the planet.
The frothing beast generates
the thundering sound and power
of thousands
of galloping horses.
[water roaring]
Even world-class surfers
test fate
when riding
the roaring wave.
[Pufahl] They have
a tremendous amount of force
that in some cases
can uproot trees.
So it's the force with which
these tidal bores move upriver
that makes them so dangerous.
[narrator] Since
the tide-watching day began
over 2,000 years ago,
this extraordinary bore tide
has come to be known
as the Silver Dragon.
[onlookers gasping]
[narrator] Curious spectators
are often swept away
because they stand too close
to the water's edge.
In 2011, fierce wind and rain
from Typhoon Nanmadol
amplified the bore
to an unprecedented 70 feet.
It overtook a dam...
and this unsuspecting reporter.
Nearly 20 years earlier,
in 1993, the bore tide festival
claimed 19 lives,
the river's worst disaster
on record.
[Pufahl] The tidal bore
traveling at 25 miles per hour,
which is typical of
the Qiantang River tidal bore,
would be like being hit by a car
traveling at 25 miles per hour.
[narrator] While the savage
bore tide on the Qiantang River
typically occurs once a year...
...there's another place
on the planet
where they happen every day.
At first glance,
the ocean tides on Earth
may look like the gentle
rhythmic movement of water,
but scientists
are discovering the secret
to why some tides
have a violent streak.
Just outside
of Anchorage, Alaska,
is Turnagain Arm,
a branch of Cook Inlet
that leads in
from the Gulf of Alaska
and the Pacific Ocean.
Its spectacular waters,
bordered by the majestic
Chugach Mountains,
attract beluga whales,
bald eagles, and black bears.
But twice each day,
a different kind
of creature emerges.
It's known as
the Turnagain Arm bore tide,
a wild wall of water
moving into the bay
against the current
at 15 miles per hour.
[man] In Turnagain Arm,
there's a special set
of conditions
that make it unique,
where the bore tides
happen every day.
In fact, it's the only place
in the United States
where they do happen
on a regular basis.
[narrator] The secret to forming
bores in Turnagain Arm
is its narrow,
shallow waterway,
which connects to Cook Inlet,
a much bigger body of water
with the largest tidal range
in the United States.
The difference between low
to high tide in Cook Inlet
can be as much as 35 feet.
[Lawson] Turnagain Arm itself,
because it's connected
to the deeper Cook Inlet,
when the tide comes in,
it's such a mass of water
flowing up the inlet
that it's got nowhere to go.
[narrator] At the mouth
of Turnagain Arm,
the 13-mile channel
quickly narrows to two miles
and then thins out even more
over its 40-mile length.
This abrupt narrowing
causes the incoming tide
to form a wave.
The crest, or highest point,
of the wave
travels at a faster speed
than the base of the wave,
which causes it to steepen.
A large wave front is produced,
and the bore
is brought to life.
For some thrill seekers,
the lure of a bore tide
is too hard to resist.
[Lawson] Kayakers, windsurfers,
even regular surfers
will try and take
advantage of it,
but they certainly know
the dangers associated with it.
The wave has been large enough
to take down a moose
and submerge it and eventually
drown it, so, you know,
it's definitely not something
to be trifled with.
[narrator] Bore tides
hold another deadly secret.
As they barrel onto beaches,
they deposit mud
and other sediments
that can hold people
in a death grip.
[Pufahl] If you ever try to walk
out on these tidal flats,
you'll quickly see why people
really don't do it.
The mud is almost
the consistency of toothpaste,
so you can sink
up to your waist.
So not appreciating
how fast the tides come in
and getting stuck can
potentially lead to drowning.
[narrator] Each year,
the local fire department
near Turnagain Arm makes
six to ten rescues of people
either trapped
in the mud flats
or overtaken
by the fierce tides.
[man] We've had people
that have got stuck out there
and swept off with the current
where we had to pull them off.
So there's certainly
a lot of risk involved
with being in the wrong spot
with the tide coming in.
[narrator] The bore is hardly
the only killer tide
on the planet.
Sometimes a harmless high tide
can be transformed
into a monster
by a ferocious storm.
[narrator] In some places
on Earth,
extreme tides are
a daily occurrence.
But elsewhere on the planet,
harsh weather can heighten
tidewater unexpectedly
and turn normally
tranquil shorelines
into hostile slivers
of real estate.
[Pufahl] The largest storms,
such as hurricanes,
typhoons, or cyclones,
are such low-pressure
weather phenomenon.
The less force
or the less air pressure
you have on the surface
of the ocean
allows that tidal water
to rise up even higher,
forming an especially high
storm surge.
[narrator] A storm surge
is the abnormal rise
of ocean water
above the usual high tide.
It's churned up
in advance of a big storm
like a hurricane
or typhoon.
A storm surge combined
with the water
from a normal high tide
produces a storm tide.
But when the storm surge
happens
during high tide
and a full moon,
it's a recipe
for a colossal tidal disaster.
October 29, 2012.
High storm tides generated
by Hurricane Sandy
battered the mid-Atlantic
and northeastern United States.
[Cantore] What made
Sandy so devastating
was the fact that you coupled
this huge storm surge
right at the time of high tide.
These two things
were maximized.
So you take
this eight-foot storm surge,
and you put it on top of what is
an astronomical high tide,
and, as we saw,
record-setting storm tides
in many locations up here.
[narrator] The storm tide surged
to 17 feet in some areas,
nearly the height
of a two-story building.
The excessive tidewater
got funneled in and trapped
by the low-lying narrow bays,
inlets, and islands.
In southwest Long Island,
storm tides flooded
over two miles inland.
The view from above
shows the wrath
of the killer storm tide.
As of 2012, Sandy is
the second-costliest hurricane
in recorded U.S. history.
[Cantore] The storm tide
rolls in,
it's gonna pick up
whatever floats,
and, unfortunately, boats float.
In this case, they just
piled them all up here.
[narrator] Around
the blue planet,
the waters will
continue to rage,
drawn by the pull of the moon,
churned into waves,
and unleashed by freak storms.
[Cantore] Going in
and out of high tide
makes the ocean different
for people that are out
on the beach enjoying it.
But we have to respect
the tides.