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In 479 BC, when Persian soldiers besieged

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the Greek city of Potidaea,

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the tide retreated
much farther than usual,

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leaving a convenient invasion route.

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But this wasn't a stroke of luck.

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Before they had crossed halfway,

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the water returned in a wave higher
than anyone had ever seen,

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drowning the attackers.

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The Potiidaeans believed
they had been saved

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by the wrath of Poseidon.

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But what really saved them

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was likely the same phenomenon
that has destroyed countless others:

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a tsunami.

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Although tsunamis are commonly
known as tidal waves,

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they're actually unrelated
to the tidal activity caused

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by the gravitational forces
of the Sun and Moon.

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In many ways, tsunamis are just
larger versions of regular waves.

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They have a trough and a crest,

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and consist not of moving water,

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but the movement of energy through water.

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The difference is
in where this energy comes from.

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For normal ocean waves,
it comes from wind.

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Because this only affects the surface,
the waves are limited in size and speed.

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But tsunamis are caused by energy
originating underwater,

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from a volcanic eruption,

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a submarine landslide,

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or most commonly,
an earthquake on the ocean floor

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caused when the tectonic plates
of the Earth's surface slip,

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releasing a massive amount
of energy into the water.

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This energy travels up to the surface,

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displacing water and raising it above
the normal sea level,

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but gravity pulls it back down,

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which makes the energy ripple
outwards horizontally.

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Thus, the tsunami is born,

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moving at over 500 miles per hour.

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When it's far from shore,
a tsunami can be barely detectable

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since it moves through
the entire depth of the water.

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But when it reaches shallow water,
something called wave shoaling occurs.

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Because there is less water
to move through,

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this still massive amount
of energy is compressed.

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The wave's speed slows down,

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while its height rises
to as much as 100 feet.

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The word tsunami,
Japanese for "harbor wave,"

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comes from the fact that it only seems
to appear near the coast.

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If the trough of a tsunami
reaches shore first,

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the water will withdraw
farther than normal

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before the wave hits,

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which can be misleadingly dangerous.

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A tsunami will not only drown
people near the coast,

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but level buildings and trees
for a mile inland or more,

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especially in low-lying areas.

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As if that weren't enough,
the water then retreats,

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dragging with it the newly created debris,

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and anything, or anyone,
unfortunate enough

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to be caught in its path.

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The 2004 Indian Ocean tsunami

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was one of the deadliest
natural disasters in history,

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killing over 200,000 people
throughout South Asia.

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So how can we protect ourselves
against this destructive force of nature?

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People in some areas have attempted

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to stop tsunamis
with sea walls, flood gates,

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and channels to divert the water.

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But these are not always effective.

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In 2011, a tsunami
surpassed the flood wall

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protecting Japan's Fukushima Power Plant,

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causing a nuclear disaster

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in addition to claiming over 18,000 lives.

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Many scientists and policy makers
are instead focusing on early detection,

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monitoring underwater pressure
and seismic activity,

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and establishing global
communication networks

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for quickly distributing alerts.

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When nature is too powerful to stop,

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the safest course
is to get out of its way.