In the open ocean, tsunamis may have wavelengths of up to several hundred miles and travel at speeds up to 500 mi per hr (800 km per hr), yet have wave heights of less than 3 ft (1 m), which pass unnoticed beneath a ship at sea. The period between the crests of a tsunami's waves varies from 5 min to about 1 hr. When tsunamis approach shallow water along a coast, they are slowed, causing their length to shorten and their height to rise sometimes as high as 100 ft (30 m). When they break, they often destroy piers, buildings, and beaches and take human life. The wave height as they crash upon a shore depends almost entirely upon the submarine topography offshore. Waves tend to rise to greater heights along gently sloping shores, along submarine ridges, or in coastal embayments.

There is little warning of approach; when a train of tsunami waves approaches a coastline, the first indication is often a sharp swell, not unlike an ordinary storm swell, followed by a sudden outrush of water that often exposes offshore areas as the first wave trough reaches the coast. After several minutes, the first huge wave crest strikes, inundating the newly exposed beach and rushing inland to flood the coast. Generally, the third to eighth wave crests are the largest.

Since tsunamis principally occur in the Pacific Ocean following shallow-focus earthquakes over magnitude 6.5 on the Richter scale, one of the best means of prediction is the detection of such earthquakes on the ocean floor with a seismograph network (see seismology). Tsunamis may be detected by wave gauges and pressure monitors, such as those emplaced as part of the U.S. Tsunami Warning System; established in 1949 and originally confined to the Pacific region, the system has been expanded to the Caribbean and the W North Atlantic. An early warning system for the Indian Ocean began operating in 2006. Measurement of sudden sea level changes from satellites are also used to warn of a potential tsunami.

One of the most destructive tsunamis to occur during historical times followed the explosive eruption of the volcano Krakatoa in the East Indies on Aug. 27, 1883, when over 36,000 people were killed as a result of the wave. Waves were up to 100 ft (30 m) high. Its passage was traced as far away as Panama. On Dec. 26, 2004, a 9.1–9.3 earthquake off NW Sumatra, Indonesia, caused a tsunami with waves as high as 65 ft (20 m) nearest the epicenter. At least 200,000 people are believed to have died. The waves devastated many areas in the E Indian Ocean basin, particularly the nearby coast of N Sumatra, the Andaman and Nicobar Islands, and the E and S coasts of Sri Lanka. Areas of SE India and SW Thailand were also hard hit. Deaths and destruction occurred as far away as the coasts of Somalia and Madagascar in Africa, and minor sea level changes were measured as far away as San Diego, Calif., Iquique, Chile, and Atlantic City, N.J. It is believed that a 0.6-mi-wide (1-km-wide) asteroid that struck the ocean SW of New Zealand about A.D. 1500 created a tsunami that reached heights of more than 425 ft (130 m).