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In 479 BC, when Persian soldiers besieged the Greek city of Potidaea, the tide retreated

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much farther than usual, 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, the water returned in a wave higher than anyone had

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ever seen, drowning the attackers.

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The Potidaeans believed they had been saved by the wrath of Poseidon.

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But what really saved them was likely the same phenomenon that has destroyed countless others,

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a tsunami. Although tsunamis are commonly known as tidal waves, they're actually unrelated to

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the tidal activity caused by the gravitational forces of the sun and moon. In many ways,

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tsunamis are just larger versions of regular waves. They have a trough and a crest and consist

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not of moving water, but the movement of energy through water. The difference is in where this

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energy comes from. For normal ocean waves, it comes from wind. Because this only affects the

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surface, the waves are limited in size and speed. But tsunamis are caused by energy originating

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underwater, from a volcanic eruption, a submarine landslide, or most commonly, an earthquake on the

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ocean floor, caused when the tectonic plates of the Earth's surface slip, releasing a massive

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amount of energy into the water. This energy travels up to the surface, displacing water and

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raising it above the normal sea level. But gravity pulls it back down, which makes the energy ripple

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outwards horizontally. Thus, the tsunami is born, moving at over 500 miles per hour. When it's far

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from shore, a tsunami can be barely detectable, since it moves through the entire depth of the

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water. But when it reaches shallow water, something called wave shoaling occurs. Because there is less

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water to move through, this still massive amount of energy is compressed. The wave 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, comes from the fact that it only seems to

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appear near the coast.

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If the trough of a tsunami reaches shore first, the water will withdraw farther than normal

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before the wave hits, which can be misleadingly dangerous.

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A tsunami will not only drown people near the coast, but level buildings and trees for

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a mile inland or more, especially in low-lying areas.

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Because if that weren't enough, the water then retreats, dragging with it the newly

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created debris and anything or anyone unfortunate enough to be caught in its path.

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The 2004 Indian Ocean tsunami was one of the deadliest natural disasters in history, killing

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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 to stop tsunamis with sea walls, floodgates, and channels

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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 protecting Japan's Fukushima power plant,

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causing a nuclear disaster in addition to claiming over 18,000 lives.

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Many scientists and policymakers are instead focusing on early detection, monitoring underwater

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pressure and seismic activity, and establishing global communication networks for quickly

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distributing alerts.

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When nature is too powerful to stop, the safest course is to get out of its way.