A tsunami is frequently, yet mistakenly, referred to as a "tidal wave". But tides have nothing to do with tsunamis. It's created when something displaces a large column of water.
The most frequent causes of tsunamis are earthquakes that occur in the oceans. The rise and fall of the ocean seafloor during quakes can displace a large amount of water, causing a tsunami. But large objects, such as a huge meteorite, can also cause tsunamis.
They're hard to spot at sea, because in the middle of the ocean, where water depths are hundreds of feet, the sea level might rise only a couple of inches, but when the wave reaches shallow shores, it can reach over 100 feet high.
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