exactly how many we have no idea. 至于确切数量我们不得而知

171. The canopy is particularly rich.
     树冠层是生命最繁荣的地方
172. There are monkeys, birds
     这里有猴子、鸟
173. and millions of species of insects,
     还有数百万种昆虫
174. exactly how many we have no idea.
     至于确切数量我们不得而知
175. The character of the forest changes as we descend,
     从上到下 森林的特点也在发生改变
176. becoming ever darker and damper,
     越往下就越阴暗潮湿
177. favouring different kinds of animals and plants.
     适合各种不同种类的动植物

来自《行星地球 第一季 第一集 Planet Earth S01E01 从北极到南极》第 174 行