WEBVTT

00:00:07.143 --> 00:00:10.717
In 479 BC, when Persian soldiers beseiged

00:00:10.717 --> 00:00:12.832
the Greek city of Potidaea,

00:00:12.832 --> 00:00:15.139
the tide retreated much farther than usual,

00:00:15.139 --> 00:00:17.956
leaving a convenient invasion route.

00:00:17.956 --> 00:00:19.814
But this wasn't a stroke of luck.

00:00:19.814 --> 00:00:21.335
Before they had crossed halfway,

00:00:21.335 --> 00:00:23.049
the water returned in a wave higher

00:00:23.049 --> 00:00:25.141
than anyone had ever seen,

00:00:25.141 --> 00:00:27.057
drowning the attackers.

00:00:27.057 --> 00:00:29.152
The Potiidaeans believed they had been saved

00:00:29.152 --> 00:00:31.211
by the wrath of Poseidon.

00:00:31.211 --> 00:00:32.851
But what really saved them was likely

00:00:32.851 --> 00:00:35.819
the same phenomenon that has destroyed countless others:

00:00:35.819 --> 00:00:37.105
a tsunami.

00:00:37.105 --> 00:00:39.811
Although tsunamis are commenly known as tidal waves,

00:00:39.811 --> 00:00:42.476
they're actually unrelated to the tidal activity caused

00:00:42.476 --> 00:00:45.621
by the gravitational forces of the sun and moon.

00:00:45.621 --> 00:00:48.095
In many ways, tsunamis are just larger versions

00:00:48.095 --> 00:00:49.548
of regular waves.

00:00:49.548 --> 00:00:51.329
They have a trough and a crest,

00:00:51.329 --> 00:00:53.299
and consist not of moving water,

00:00:53.299 --> 00:00:56.380
but the movement of energy through water.

00:00:56.380 --> 00:00:58.884
The difference is in where this energy comes from.

00:00:58.884 --> 00:01:00.188
For normal ocean waves,

00:01:00.188 --> 00:01:01.792
it comes from wind.

00:01:01.792 --> 00:01:03.572
Because this only affects the surface,

00:01:03.572 --> 00:01:06.018
the waves are limited in size and speed.

00:01:06.018 --> 00:01:07.622
But tsunamis are caused by energy

00:01:07.622 --> 00:01:09.601
originating underwater,

00:01:09.601 --> 00:01:10.833
from a volcanic eruption,

00:01:10.833 --> 00:01:12.501
a submarine landslide,

00:01:12.501 --> 00:01:13.896
or most commonly,

00:01:13.896 --> 00:01:15.609
an earthquake on the ocean floor

00:01:15.609 --> 00:01:18.792
caused when the tectonic plates of the Earth's surface slip,

00:01:18.792 --> 00:01:21.987
releasing a massive amount of energy into the water.

00:01:21.987 --> 00:01:24.152
This energy travels up to the surface,

00:01:24.152 --> 00:01:26.117
displacing water and raising it above

00:01:26.117 --> 00:01:27.954
the normal sea level,

00:01:27.954 --> 00:01:29.777
but gravity pulls it back down,

00:01:29.777 --> 00:01:33.134
which makes the energy ripple outwards horizontally.

00:01:33.134 --> 00:01:34.785
Thus, the tsunami is born,

00:01:34.785 --> 00:01:37.577
moving at over 500 miles per hour.

00:01:37.577 --> 00:01:38.953
When it's far from shore,

00:01:38.953 --> 00:01:40.991
a tsunami can be barely detectable

00:01:40.991 --> 00:01:43.673
since it moves through the entire depth of the water.

00:01:43.673 --> 00:01:45.083
But when it reaches shallow water,

00:01:45.083 --> 00:01:47.534
something called wave shoaling occurs.

00:01:47.534 --> 00:01:49.788
Because there is less water to move through,

00:01:49.788 --> 00:01:53.271
this still massive amount of energy is compressed.

00:01:53.271 --> 00:01:55.115
The wave's speed slows down,

00:01:55.115 --> 00:01:58.550
while its height rises to as much as 100 feet.

00:01:58.550 --> 00:01:59.563
The word tsunami,

00:01:59.563 --> 00:02:01.631
Japanese for "harbor wave,"

00:02:01.631 --> 00:02:03.759
comes from the fact that it only seems to appear

00:02:03.759 --> 00:02:05.252
near the coast.

00:02:05.252 --> 00:02:07.882
If the trough of a tsunami reaches shore first,

00:02:07.882 --> 00:02:09.919
the water will withdraw farther than normal

00:02:09.919 --> 00:02:13.070
before the wave hits, which can be misleadingly dangerous.

00:02:13.070 --> 00:02:16.319
A tsunami will not only drown people near the coast,

00:02:16.319 --> 00:02:20.134
but level buildings and trees for a mile inland or more,

00:02:20.134 --> 00:02:22.164
especially in low-lying areas.

00:02:22.164 --> 00:02:23.449
As if that weren't enough,

00:02:23.449 --> 00:02:24.963
the water then retreats,

00:02:24.963 --> 00:02:27.212
dragging with it the newly created debris,

00:02:27.212 --> 00:02:29.737
and anything, or anyone, unfortunate enough

00:02:29.737 --> 00:02:31.478
to be caught in its path.

00:02:31.478 --> 00:02:33.913
The 2004 Indian Ocean tsunami

00:02:33.913 --> 00:02:37.000
was one of the deadliest natural disasters in history,

00:02:37.000 --> 00:02:40.177
killing over 200,000 people throughout South Asia.

00:02:40.177 --> 00:02:42.082
So how can we protect ourselves

00:02:42.082 --> 00:02:44.646
against this destructive force of nature?

00:02:44.646 --> 00:02:46.379
People in some areas have attempted

00:02:46.379 --> 00:02:50.212
to stop tsunamis with sea walls, flood gates,

00:02:50.212 --> 00:02:51.722
and channels to divert the water.

00:02:51.722 --> 00:02:53.279
But these are not always effective.

00:02:53.279 --> 00:02:56.157
In 2011, a tsunami surpassed the flood wall

00:02:56.157 --> 00:02:58.878
protecting Japan's Fukushima Power Plant,

00:02:58.878 --> 00:03:01.471
causing a nuclear disaster in addition to claiming

00:03:01.471 --> 00:03:04.142
over 18,000 lives.

00:03:04.142 --> 00:03:06.470
Many scientists and policy makers are instead

00:03:06.470 --> 00:03:08.569
focusing on early detection,

00:03:08.569 --> 00:03:11.754
monitoring underwater pressure and seismic activity,

00:03:11.754 --> 00:03:14.117
and establishing global communication networks

00:03:14.117 --> 00:03:15.737
for quickly distributing alerts.

00:03:15.737 --> 00:03:18.469
When nature is too powerful to stop,

00:03:18.469 --> 00:03:21.879
the safest course is to get out of its way.