The least known parts of it are its great depths. 这之中最不为人知的区域是海洋深处

160. ..and they do so in astounding numbers.
     而且规模惊人
161. Tens of thousands of rays in a single shoal.
     数以万计的蝠鲼聚集在一起
162. Vast areas of our ocean are yet to be explored.
     海洋中还有广阔的区域尚未探索
163. The least known parts of it are its great depths.
     这之中最不为人知的区域是海洋深处
164. Only highly-specialised vessels can take you there.
     只有高度专业化的潜艇才能抵达
165. As you descend, the sunlight rapidly fades.
     随着越潜越深 阳光会迅速减弱
166. 200m down, you enter the twilight zone...
     200米深处 是一片黄昏世界

来自《行星地球 第三季 第二集 Planet Earth S03E02 海洋》第 163 行