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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, but what really

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saved them was likely the same phenomenon that has destroyed countless others, a tsunami.

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Although tsunamis are commonly known as tidal waves, they're actually unrelated to the tidal

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activity caused by the gravitational forces of the sun and moon. In many ways, tsunamis are just

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larger versions of regular waves. They have a trough and a crest, and consist not of moving

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water, but the movement of energy through water. 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, 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, which makes the energy ripple outwards horizontally.

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Thus, the tsunami is born, 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, this still massive amount of energy is compressed.

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The wave's speed slows down 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 appear near the coast.

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If the trough of a tsunami reaches shore first, the water will withdraw farther than normal 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 a mile inland or more, especially in low-lying areas.

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

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debris and anything or anyone unfortunate enough to be caught in its path. The 2004

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

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killing over 200,000 people throughout South Asia. So how can we protect ourselves against

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this destructive force of nature? People in some areas have attempted to stop tsunamis with sea

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walls, floodgates, and channels to divert the water. But these are not always effective. In 2011,

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a tsunami surpassed the flood wall protecting Japan's Fukushima power plant, causing a nuclear

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disaster in addition to claiming over 18,000 lives. Many scientists and policymakers are

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instead focusing on early detection, monitoring underwater pressure and seismic activity,

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and establishing global communication networks for quickly distributing alerts.

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