Earthquakes are one of the most powerful events on the earth. When large sections of underground rock break and move suddenly, and earthquake occurs. This causes the ground to shake back and forth. Small earthquakes do not cause much damage, but large ones do. Some earthquakes have caused buildings and bridges to fall. Others have caused rivers to change their paths. Earthquakes near mountains and cliffs can cause landslides that cover up the houses and roads below. A large earthquake under the ocean can cause giant waves that flood the seashore. When large earthquakes occur in a city, there is danger of fire from broken gas and electric lines. Damaged roads make it difficult for rescue workers to help people. Scientists are trying to find ways to predict when an earthquake will happen so that people can be warned ahead of time.