Witnesses to tsunamis report they make a roaring sound as they approach, similar to the sound of a freight train or a jet airplane. Tsunamis are triggered by underwater earthquakes, which are reported to make similar sounds. A hydrophone captured the sound of the 9.0 earthquake that struck Japan in 2011 and caused a tsunami across the Pacific Ocean, which was like the sound of a rocket taking off.
The roaring sound made by an approaching tsunami is only one of several warning signs. Any earthquake in a tsunami zone should cause concern, as the speed of tsunami waves is high. Water often recedes from the shore as a tsunami approaches, exposing the floor of the ocean. When this occurs, the tsunami itself is likely to arrive within five minutes, so observers are advised not to go down to the ocean to see what is happening but to seek high ground immediately.
Other abnormal ocean activity and the approach of a wall of water are also cause for concern. An approaching tsunami often looks like a river or avalanche moving at high speed across the surface of the ocean. Sometimes it takes the form of a rapidly rising tide. Tsunamis typically arrive as a series of waves, so the departure of one wave doesn't mean the danger has passed.