海にはなぜ波があるの？

The ocean is never still. Whether we look out from the beach or from a boat, we expect to see waves on the horizon. Waves are created by energy passing through water, which makes the water move in a circular motion. But the water itself doesn't actually travel with the wave. Waves carry energy, not water, across the ocean, and if nothing gets in their way, they can travel across an entire ocean basin. Most waves are caused by wind. Wind-driven waves, or surface waves, are created by friction between the wind and the surface of the water. As the wind blows across the ocean or a lake, the constant disturbance creates a wave crest. These waves are found all over the open ocean and along the coast. More dangerous waves can be caused by severe weather such as hurricanes. The strong winds and low pressure of these storms cause a storm surge: a series of long waves that form far from shore in deep water and grow stronger as they move closer to land. Other dangerous waves are caused by underwater events that suddenly move large amounts of water, such as earthquakes, landslides or volcanic eruptions. These very long waves are called tsunamis. Storm surges and tsunamis are not the kind of waves you picture crashing onto the beach. Instead, they roll in like a massive rise in sea level and can reach far inland. The pull of gravity from the Sun and the Moon also causes waves. These waves are the tides. Many people think "tidal wave" is just another word for a tsunami, but this is a misunderstanding. Tsunamis have nothing to do with the tides, and they can happen at any time, whether the tide is high or low.