During its journey from the surface 在地面上流淌时

56. its course is altered.
    就会改变路线
57. Once underground,
    一旦进入地下
58. the water takes on a new, more erosive power.
    水便有了一股新的、更强的侵蚀力
59. During its journey from the surface
    在地面上流淌时
60. the water absorbed carbon dioxide from the soil
    水从土壤中吸收了二氧化碳
61. making it mildly acidic.
    成为弱酸性
62. And over millions of years this acid eats away the limestone
    石灰岩经过几百万年的酸蚀作用

来自《行星地球 第一季 第四集 Planet Earth S01E04 洞穴》第 59 行