that an early warning system is essential. 所以即时预警设备是最重要的

277. Explorers entering this dangerous cave
     探险家们进入这个危险的山洞时
278. must wear respirators and carry monitors.
     必须戴上防毒面具 并携带通话器
279. Poisonous gases rise to fatal levels so quickly
     有毒气体很快就会达到致命的程度
280. that an early warning system is essential.
     所以即时预警设备是最重要的
281. Bats survive by staying close to the skylights,
     蝙蝠之所以能生存是因为 它们始终呆在天窗附近
282. but venturing deep into the cave is very dangerous indeed.
     但是深入洞穴必然极度危险
283. The source of these toxic fumes lies several miles below.
     这些毒气的源头位于地下几英里处

来自《行星地球 第一季 第四集 Planet Earth S01E04 洞穴》第 280 行