literally. 绝对如此

186. To avoid getting lost
     为了避免迷路
187. divers carry with them a spool of string.
     潜水员必须随身携带一捆轴线
188. It becomes their lifeline -
     这成了它们的救生索
189. literally.
     绝对如此
190. The string also doubles as a measuring tape -
     这根线同时也是测量尺
191. a technique that has been used here, in Mexico, to chart the largest underwater cave in the world -
     在墨西哥的这个地方 这种科技被用来 测绘世界上最大的地下水洞
192. all 100 miles of it.
     全长达100多英里

来自《行星地球 第一季 第四集 Planet Earth S01E04 洞穴》第 189 行