translation.en: Karstic regions cover almost 20 percent of the continents' surface, and we know actually that speleologists in the last 50 years have explored roughly 30,000 kilometers of cave passages around the world, which is a big number.
translation.zh: 喀斯特地区几乎覆盖了陆地表面的20%， 在过去的50年里，洞穴学家们 在全世界探索了差不多3万公里的洞穴通道， 这是个了不起的数字。