The power of water can be so destructive it can kill thousands of people in seconds with little warning.
The U.S. is lucky that it knew about a threat of a tsunami and had hours to prepare and evacuate following Friday's massive earthquake in Japan.
Japan was inundated with as much as 30 feet of water shortly after the 8.9-magnitude quake. Residents had about a 15-minute warning to get to higher ground. The death toll is already in the hundreds and still rising. Tsunami warnings were issued for the entire Pacific basin.
When a powerful earthquake moves the seafloor and displaces water, it spawns a tsunami, a series of waves that can travel through the water for thousands of miles at speeds up to 600 mph.
That's as fast as a jetliner.
We know how and why these things happen, but what scientists don't know is how big the waves will be when they arrive, how far apart they will be and which of the series of waves will be the largest. Often, the first tsunami wave isn't the biggest.
Tsunamis can't be seen from the air or felt on a ship. They are different from wind-whipped and tidal waves. Tsunami waves are shallow water waves, so there isn't a great difference between the average height of the deep open water and the height of a tsunami.
That changes when the tsunami reaches shallow water and land. The wave grows taller depending on the coastline. The waves can get especially high in harbors and bays as water gets funneled and lifted up from the seafloor.
It can be compared to the storm surge in a hurricane, but on a much larger, faster and forceful scale.
Friday's quake was the fifth most-powerful earthquake since 1900, according to the U.S. Geological Survey. It is natural to compare this disaster to similar events in history. Who can forget the horrific Sumatran earthquake and tsunami in 2004 that killed nearly 300,000 people? The Chilean earthquake and tsunami last year is also fresh in most people's minds.
But it's been decades since the U.S. has seen casualties as a result of a tsunami. In 1964, 11 people died in Crescent City, California, after a tsunami off Alaska. In 1960, 61 people in Hawaii died after a 35-foot tsunami followed an earthquake in Chile. In 1946, 159 people were killed in Hilo, Hawaii, after an Alaskan quake.
It's clear the U.S. will face this threat again, and there's no question deaths will occur when a big enough tsunami develops. It's scary to think about such things.
Since we can't predict when an earthquake will happen, we don't know if it will be days, weeks, months or years before another tsunami hits.
All we can do is plan "what if" scenarios and try to educate people that evacuating inland to higher ground is critical, and sometimes you only have time to evacuate "up" or to a higher floor in the building.