Very high waves are destructive when they strike the land. Fortunately, this seldom happened. One reason is that out at sea, waves moving in one direction almost always run into waves moving in a different direction. The two sets of waves tend to cancel each other out. Another reason is that water is shallower near the shore. As a wave gets closer to land, the shallow bottom helps reduce its strength. But the power of waves striking the shore can still be very great. During a winter gale(大风), waves sometimes strike the shore with the force of 6,000 pounds for each square foot. That means a wave, 25 feet high and 500 feet along its face, may strike the shore with a force of 75 million pounds. Yet the waves, no matter how big or how violent, affect only the surface of the sea. During the most violent storms, the water a hundred fathoms(600 feet) beneath the surface is just as calm as on a day without a breath of wind. According to the passage, destruction caused by high waves occurs ______.