Tsunami is a very large ocean wave that is caused by an underwater earthquake or volcanic eruption and often causes extreme destruction when it strikes land. Tsunami waves do not resemble normal sea waves, because their wavelength is far longer. A tsunami resembles a rapidly rising tide, and for this reason they are often referred to as tidal waves. Tsunamis generally consist of a series of waves with periods ranging from minutes to hours, arriving in a so-called wave train. Tsunamis can have heights of up to 98 ft and reach speeds of 589 miles per hour. Photo originally captioned: "Tsunami inundating Hilo, Hawaii. Note man on pier. This disaster provided impetus for C&GS to establish tsunami warning system."