The term 'gradually break down' can be summarized by the word 'ERODE, ' which has five letters. Erosion is a natural process in which soil, rocks, and other materials are worn away by wind, water, or other natural forces. Over time, this process can cause gradual breakdown of landforms, such as mountains, valleys, and coastlines. Erosion can also occur due to human activities, such as deforestation, overgrazing, and construction.