What happens underwater to cause a tsunami?

Answer 1

The most common underwater causes of tsunami are earthquakes and volcanoes. The event causes a large column of water to be displaced and moved out from the source. See the related question about the causes of tsunami for more detail about this question and about other causes of tsunami that may not start underwater.

Answer 2

A tsunami is typically caused by underwater earthquakes, volcanic eruptions, or landslides that displace a large volume of water. This displacement creates a series of powerful waves that travel rapidly across the ocean, eventually reaching shorelines and causing widespread destruction.