Most of the world's caves are found within it 世界上大部分洞穴内都存在石灰岩

40. finding a regular and reliable source of food.
    即找到长期而可靠的食物资源
41. One kind of rock makes this whole underground world possible -
    这些地底世界之所以会存在 都是因为一种岩石
42. limestone.
    石灰岩
43. Most of the world's caves are found within it
    世界上大部分洞穴内都存在石灰岩
44. and it covers nearly 10 percent of the earth's surface.
    它覆盖了地球表面将近10%的面积
45. Limestone is composed of minerals derived from marine shells and corals,
    石灰岩由海螺和 珊瑚中的矿物沉积后形成
46. so although this rocky escarpment in the United States is now hundreds of metres above sea level
    所以 尽管美国境内的这座石壁 如今矗立在海平面以上几百米高的地方

来自《行星地球 第一季 第四集 Planet Earth S01E04 洞穴》第 43 行