WEBVTT

00:00:07.143 --> 00:00:10.660
In 479 BC, when Persian soldiers besieged

00:00:10.684 --> 00:00:12.636
the Greek city of Potidaea,

00:00:12.660 --> 00:00:14.943
the tide retreated
much farther than usual,

00:00:14.968 --> 00:00:17.528
leaving a convenient invasion route.

00:00:17.552 --> 00:00:19.552
But this wasn't a stroke of luck.

00:00:19.576 --> 00:00:21.101
Before they had crossed halfway,

00:00:21.125 --> 00:00:24.951
the water returned in a wave higher
than anyone had ever seen,

00:00:24.975 --> 00:00:26.766
drowning the attackers.

00:00:26.790 --> 00:00:29.010
The Potiidaeans believed
they had been saved

00:00:29.034 --> 00:00:30.709
by the wrath of Poseidon.

00:00:30.733 --> 00:00:32.240
But what really saved them

00:00:32.264 --> 00:00:35.795
was likely the same phenomenon
that has destroyed countless others:

00:00:35.819 --> 00:00:36.915
a tsunami.

00:00:36.939 --> 00:00:39.787
Although tsunamis are commonly
known as tidal waves,

00:00:39.811 --> 00:00:42.452
they're actually unrelated
to the tidal activity caused

00:00:42.476 --> 00:00:45.597
by the gravitational forces
of the Sun and Moon.

00:00:45.621 --> 00:00:49.239
In many ways, tsunamis are just
larger versions of regular waves.

00:00:49.263 --> 00:00:51.137
They have a trough and a crest,

00:00:51.161 --> 00:00:53.275
and consist not of moving water,

00:00:53.299 --> 00:00:55.422
but the movement of energy through water.

00:00:56.228 --> 00:00:58.709
The difference is
in where this energy comes from.

00:00:58.733 --> 00:01:01.524
For normal ocean waves,
it comes from wind.

00:01:01.548 --> 00:01:05.814
Because this only affects the surface,
the waves are limited in size and speed.

00:01:05.838 --> 00:01:09.225
But tsunamis are caused by energy
originating underwater,

00:01:09.249 --> 00:01:10.809
from a volcanic eruption,

00:01:10.833 --> 00:01:12.477
a submarine landslide,

00:01:12.501 --> 00:01:15.396
or most commonly,
an earthquake on the ocean floor

00:01:15.420 --> 00:01:18.768
caused when the tectonic plates
of the Earth's surface slip,

00:01:18.792 --> 00:01:21.963
releasing a massive amount
of energy into the water.

00:01:21.987 --> 00:01:24.128
This energy travels up to the surface,

00:01:24.152 --> 00:01:27.930
displacing water and raising it above
the normal sea level,

00:01:27.954 --> 00:01:29.753
but gravity pulls it back down,

00:01:29.777 --> 00:01:32.686
which makes the energy ripple
outwards horizontally.

00:01:32.710 --> 00:01:34.761
Thus, the tsunami is born,

00:01:34.785 --> 00:01:37.376
moving at over 500 miles per hour.

00:01:37.400 --> 00:01:40.967
When it's far from shore,
a tsunami can be barely detectable

00:01:40.991 --> 00:01:43.649
since it moves through
the entire depth of the water.

00:01:43.673 --> 00:01:47.510
But when it reaches shallow water,
something called wave shoaling occurs.

00:01:47.534 --> 00:01:49.764
Because there is less water
to move through,

00:01:49.788 --> 00:01:53.158
this still massive amount
of energy is compressed.

00:01:53.182 --> 00:01:55.091
The wave's speed slows down,

00:01:55.115 --> 00:01:58.090
while its height rises
to as much as 100 feet.

00:01:58.114 --> 00:02:01.607
The word tsunami,
Japanese for "harbor wave,"

00:02:01.631 --> 00:02:04.678
comes from the fact that it only seems
to appear near the coast.

00:02:04.702 --> 00:02:07.858
If the trough of a tsunami
reaches shore first,

00:02:07.882 --> 00:02:09.930
the water will withdraw
farther than normal

00:02:09.954 --> 00:02:11.120
before the wave hits,

00:02:11.144 --> 00:02:13.161
which can be misleadingly dangerous.

00:02:13.185 --> 00:02:16.208
A tsunami will not only drown
people near the coast,

00:02:16.232 --> 00:02:19.554
but level buildings and trees
for a mile inland or more,

00:02:19.578 --> 00:02:21.933
especially in low-lying areas.

00:02:21.957 --> 00:02:24.939
As if that weren't enough,
the water then retreats,

00:02:24.963 --> 00:02:26.979
dragging with it the newly created debris,

00:02:27.003 --> 00:02:29.713
and anything, or anyone,
unfortunate enough

00:02:29.737 --> 00:02:31.144
to be caught in its path.

00:02:31.168 --> 00:02:33.889
The 2004 Indian Ocean tsunami

00:02:33.913 --> 00:02:36.572
was one of the deadliest
natural disasters in history,

00:02:36.596 --> 00:02:40.236
killing over 200,000 people
throughout South Asia.

00:02:40.260 --> 00:02:44.622
So how can we protect ourselves
against this destructive force of nature?

00:02:44.646 --> 00:02:46.355
People in some areas have attempted

00:02:46.379 --> 00:02:49.397
to stop tsunamis
with sea walls, flood gates,

00:02:49.421 --> 00:02:51.169
and channels to divert the water.

00:02:51.193 --> 00:02:52.861
But these are not always effective.

00:02:52.885 --> 00:02:56.494
In 2011, a tsunami
surpassed the flood wall

00:02:56.518 --> 00:02:58.728
protecting Japan's Fukushima Power Plant,

00:02:58.752 --> 00:03:00.858
causing a nuclear disaster

00:03:00.882 --> 00:03:03.430
in addition to claiming over 18,000 lives.

00:03:04.235 --> 00:03:08.545
Many scientists and policy makers
are instead focusing on early detection,

00:03:08.569 --> 00:03:11.730
monitoring underwater pressure
and seismic activity,

00:03:11.754 --> 00:03:14.093
and establishing global
communication networks

00:03:14.117 --> 00:03:15.713
for quickly distributing alerts.

00:03:16.291 --> 00:03:18.445
When nature is too powerful to stop,

00:03:18.469 --> 00:03:21.105
the safest course
is to get out of its way.