they will all drop to the water surface, 便会一起落到水面

147. But these spiralling columns hundreds if metres high
     这些高达数百米的螺旋型“烟”柱
148. are mating flies.
     其实是正在交配的幽蚊
149. Once the flies have mated,
     幽蚊完成交配之后
150. they will all drop to the water surface,
     便会一起落到水面
151. release their eggs and die.
     产下卵 然后死去
152. Malawi may look like an inland sea,
     马拉维湖看起来像大海一样辽阔
153. but it's dwarfed by the world's largest lake -
     但它远比不上世界上最大的湖泊

来自《行星地球 第一季 第三集 Planet Earth S01E03 淡水》第 150 行