limestone. 石灰岩

39. these glow worms have solved the biggest challenge that permanent cave dwellers face -
    这些萤火虫成功地解决了 终生穴居动物面临的最大难题
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

来自《行星地球 第一季 第四集 Planet Earth S01E04 洞穴》第 42 行