What do you know about the sea? The fist thing to remember is that the sea is very big. Look at map of the world and you'll find that there is less land than sea. The sea covers three-quarters of the world. The sea is full of waves. Waves are beautiful to look at, but they can destroy (破)ships at sea, as well as houses and buidings near the shore. What causes waves? Most waves are caused by winds blowing(吹)over the surface of the water. The sun heats the earth, causing the air to rise and the winds to blow. The winds blow across the sea, pushing litle waves into bigger and bigger ones. The size of a wave depends on how strong the wind is, how long it blows and how large the body of water is. In a small bay, waves will never build up. But at sea, the wind can build up very big and powerful waves. A rule says that the height of a wave (in meters) will usually be no more than one-tenth of the wind speed (in kilometers). In other words, when the wind is blowing at 120 kilometers per hour, most waves will be about 12 meters high. Of course, somewaves may combine to form big waves that are much higher. In 1933, the largest measured wave in history was reported. It rose in the Pacific Ocean to a height of 34meters.