The giant waves are more dangerous on flat shores than on steep ones. They usually range from 20 to 60 feet in height, but when they pour into a V shaped inlet or harbor they may rise to mountainous proportions.

Generally the first salvo of a tsunami is a rather sharp swell, not different enough from an ordinary wave to alarm casual observers. This is followed by a tremendous suck of water away from the shore as the first great trough arrives. Reefs are left high and dry, and the beaches are covered with stranded fish. At Hilo large numbers of people ran out to inspect the amazing spectacle of the denuded beach. Many of them paid for their curiosity with their lives, for some minutes later the first giant wave roared over the shore. After an earthquake in Japan in 1793 people on the coast at Tugaru were so terrified by the extraordinary ebbing of the sea that they scurried to higher ground. When a second quake came, they dashed back to the beach, fearing that they might be buried under landslides. Just as they reached the shore, the first huge wave crashed upon them.

A tsunami is not a single wave but a series. The waves are separated by intervals of 15 minutes to an hour or more (because of their great length), and this has often lulled people into thinking after the first great wave has crashed that it is all over. The waves may keep coming for many hours. Usually the third to the eighth waves in the series are the biggest.