Shortly after 7 a.m. on April 1, 1946, the first devastating tsunami waves hit Hilo, Hawaii. As one survivor, Kapua Wall Heuer, described it, “I looked out … and saw this great big black wall coming in…. The noise was terrific…. And then you heard the screaming … people were stomping, trying to reach earth, trying to get out. Dogs swimming around. Then came the crash…. [I]t hit buildings, the lighthouse, and the railroad track and everything. And the roar. And I said, ‘Oh, that's good-bye to Hilo.’” Waves 8 meters (25 feet) high smashed every house facing Hilo Bay. Survivors struggled for days to retrieve the bodies of the victims and to clear the massive amounts of debris.

Tsunami (tsoo NAH mee) is a Japanese word that means harbor wave, though scientists now know that tsunamis do not originate in harbors. Furthermore, though tsunamis have often been called tidal waves, they have nothing to do with tides. Rather, a tsunami is a series of waves that forms when a strong disturbance—most often an earthquake—occurs on the ocean floor. The waves travel outward from the point of the disturbance. On the surface of the water, the waves spread as a succession of expanding circles, like the waves produced when a pebble is thrown into a pond.

The cause of the tsunami that struck Hilo in 1946 was an earthquake that occurred about 145 kilometers (90 miles) off Unimak Island, one of the Aleutian Islands in Alaska. The waves reached Unimak 48 minutes after the earthquake, leveling a lighthouse and killing five members of the United States Coast Guard who were stationed there. Waves that reached as high as 42 meters (135 feet) above sea level inundated the coast.

Meanwhile, on the opposite side of the circle of waves, the tsunami sped across the ocean at 800 kilometers (500 miles) per hour. It reached Hawaii less than five hours after the earthquake occurred, killing 159 people and causing $26 million in property damage.

After the catastrophe of 1946, the United States and other countries took steps to better prepare themselves for tsunamis. National and international agencies set up monitoring systems to help them spot earthquakes and other underwater disturbances as they occur and to determine which disturbances might generate tsunamis. Local governments developed warning systems that can alert people to an impending tsunami in time for them to leave a dangerous area.

But even with all this preparation, tsunamis remain deadly. During the 1990's alone, 10 tsunamis killed more than 4,000 people. So scientists throughout the world continue to study the causes of tsunamis. By looking at past tsunamis, researchers are learning where and how often tsunamis are most likely to occur. By measuring the heights of previous tsunami waves, they are able to estimate flooding limits at specific coastal locations. The main objectives of all this research are to develop ways to detect the deadly waves more quickly and more accurately and to forecast where the waves will hit and how big they will be.