] Rain forests cover only about six percent of the earth's surface, but they are very important to the earth. What is a rain forest? It is an area of land that gets a lot of rainfall and is mostly covered by tall, old trees. Some rain forests get up to 33 feet (10 meters) of rain each year. Some of their trees are thousands of years old. Most of the world's rain forests are in Africa, Asia, Australia, Central America, and South America. More than half of the world's plant and animal species live in rain forests or originally came from rain forests. Scientists continue to discover plants in the rain forests that have medicinal value (useful for treating medical problems). In fact, over 25 percent of the medicines we have come from rain forest plants, and there are still a lot of plants to find.