Although commonly referred to as tidal waves, tsunamis are not caused by tides. A tsunami is a series of fast-moving waves created by a disturbance above or below sea level. Although earthquakes are the most common cause of tsunamis, volcanic eruptions and landslides can also trigger a tsunami. A tsunami can occur in any large body of water, however they typically occur in the Pacific Ocean. In open water, a tsunami travels at speeds of up to 800 km per hour, with wavelengths several hundred kilometres long but wave heights under one metre. When it approaches land, however, a tsunami slows, its wave length compressing and its height increasing. In 2004, a catastrophic tsunami resulting from an earthquake in the Indian Ocean claimed more than 200,000 lives with 20-metres-high waves. While scientists cannot predict when a tsunami will occur, seismologists monitoring submarine earthquakes can forecast a tsunami’s arrival and impact using wave theory and measurement technology, thus enabling authorities to issue tsunami warnings.