Coasts are constantly shifting as sea levels change and waves erode the land. In some parts of the world, coasts are transforming so rapidly that the changes are observable each year. This poses a threat to homes and buildings along the coasts.
Intriguing coastal features such as arches and pillars are formed when soft rocks are eroded by the waves, forming hollow caves. When there is a cave on each side of a large, soft rock, the caves may eventually be eroded until they form the inside of an arch. If the top of the arch collapses, a lone pillar is left behind.
Coasts also change according to the sea level, which has risen and fallen repeatedly over the past few million years. During an ice age, sea levels drop because so much of the oceans' water is frozen. When the ice age ends and temperatures rise, the ice melts, leading to a big rise in sea levels.
Due to coastal erosion and rising sea levels, hundreds, if not thousands, of coastal cities may experience severe problems in the next 100 years throughout the world. Flooded coasts change shape, sometimes permanently, and coastal flooding is considered a huge threat to coastal civilization. Entire cities could be lost over time to rising sea levels.