Tsunamis are large, destructive waves that are caused by the sudden movement of a large area of the sea floor. Tsunamis are often incorrectly called tidal waves, but unlike regular ocean tides they are not caused by the gravitational pull of the moon and sun. Most tsunamis are caused by 'earthquakes', some are caused by submarine 'landslides', a few are caused by submarine 'volcanic eruptions' and on rare occasions they are caused by a large meteorite impact in the ocean. The 'December 26, 2004 magnitude 9.0 earthquake near Sumatra' produced the largest trans-oceanic tsunami in over 40 years, and killed more people than any tsunami in recorded history. The 'Krakatau' volcanic eruption of 1883 generated giant waves reaching heights of 125 feet above sea level, killing thousands of people and wiping out numerous coastal villages. While tsunami means "harbor wave" in Japanese, a tsunami is actually a series of large waves created by the sudden movement of the seafloor. The energy generated by the earthquake or other event is transmitted through the water as a large train of waves, but the movement of these waves is very different from the movement of waves generated by wind. NASA's Physics Behind the Wave explains the structure of tsunamis. Tsunamis can travel rapidly across oceans, causing destruction far from the location where they were generated. All oceanic regions of the world experience tsunamis, although tsunamis in the Atlantic, Mediterranean, and Caribbean tend to be smaller and less destructive than those in the Pacific and Indian Oceans. About 90 percent of recorded tsunamis occur in the Pacific Ocean. The reasons for this lie in the geologic structure of the Pacific basin - the ocean is surrounded by a geologically active series of mountain chains, deep ocean, trenches, and island arcs, sometimes called "the ring of fire." The earthquakes and volcanic eruptions that occur in the ring of fire are the source of many tsunamis. The height of a tsunami in the deep ocean is small - usually about 1 foot - and they cannot be seen or felt by ships at sea. The distance between wave crests can be more than 100 miles. The speed at which the tsunami travels decreases as water depth decreases. In the deep waters of the mid-Pacific, a tsunami can reach a speed of more than 500 miles per hour, but in the shallow waters near land the speed drops to 100 miles per hour or less. As tsunamis reach shallow water the height of the waves increases dramatically, and can reach 100 feet or more. These huge waves can wash far inland, carrying large amounts of debris, destroying buildings and other structures, causing widespread flooding, and dramatically altering shorelines. Most tsunamis consist of a series of waves, and the first wave to reach shore may not be the largest. Locally generated tsunamis may reach a shoreline with only a few minutes warning, while distant events may allow several hours warning. Warning signs of an approaching tsunami include a strong earthquake felt near the shore or a rapid fall in the water level - like a sudden and extremely low tide. Either of these signs should be taken as a warning that a tsunami is imminent and that coastal areas should be immediately evacuated. In addition, many coastal areas have tsunami alert systems that sound sirens or provide information through local media. The United States has a tsunami warning system in place for the west coast, Hawaii, and Alaska. The 'West Coast and Alaska Tsunami Warning Center' (WCATWC) in Palmer, Alaska, provides information for Alaska, Washington, Oregon, California, and British Columbia. WCATWC also provides online 'tsunami safety advice'. Information for the remaining portions of the Pacific basin is supplied by the 'Pacific Tsunami Warning Center' in Hawaii. The tsunami warning centers issue two types of bulletins to advise of a possible approaching tsunami. A Tsunami Watch Bulletin is released when an earthquake occurs with a magnitude of 6.75 or greater on the Richter scale. A Tsunami Warning Bulletin is released when information from tidal stations indicates that a potentially destructive tsunami exists. Tidal stations record information about the water around them and issue a warning when characteristics of the sea begin to match those of a potential tsunami. While we can't prevent tsunamis, we can take steps to lessen their impact. Those who live in or visit tsunami-prone areas should know the warning signs of an approaching tsunami, and what to do when a tsunami is imminent