The presentation of Korean cave

There are some 1,000 natural caves in Korea. Most caves on the mainland are made of limestone, whereas most of the caves on Cheju Island are volcanic in origin. The caves on Cheju, in particular, are internationally renowned for their huge size and scientific value. By contrast, the caves on the mainland are not as big, but their unique shapes and formations sti1l attract the attention of international speleologists.(omitted)