The waves that break on the seashore are usually caused by winds blowing across the open sea. During storms, waves reach 12m (39 ft) or more in height. The highest wave recorded in the open sea was 34m (112ft) high.
Other large and often destructive waves, called tsunamis, are triggered off by earthquakes or volcanic eruption. They are low, fast waves, with immense energy. As tsunamis near land, they lose speed and their energy is converted into an increase in height. The highest recorded tsunami appeared off the Japanese Ryukyu Island in 1971. It was 85m (278 ft) high. In 1883, the volcanic explosion at Krakatoa set off tsunamis that drowned 36,000 people on nearby islands.