Once underground, 一旦进入地下

54. Rivers that flow over limestone often seem to completely disappear.
    流经石灰岩的河川 到最后似乎总是无影无踪
55. When the water reaches the more resistant bed of limestone
    当水流遇到更坚硬的石灰岩床时
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

来自《行星地球 第一季 第四集 Planet Earth S01E04 洞穴》第 57 行