蜜芽传媒

This cave is home to ten million Mexican free-tailed bats. The piles of their droppings can be 50 feet thick and that's dangerous, because bat droppings give off toxic ammonia gas. A quarter of a mile below the surface the air is unbreathable. Without masks, in 10 minutes humans would black out. In 20, they'd never wake up. But for bats, this cave provides almost total protection as no predators can stand it down here for long and the temperature and humidity are just perfect. There are around 200 bats packed into every square foot, turning the ceiling into a fur-lined mass of bat bodies. They make this one of the most overcrowded, disease-ridden, toxic black holes on Earth. So the bats have to be tough. They have an internal system that neutralises the deadly ammonia gas, and they can even survive an attack of rabies. Of course the bats only spend the day down here, at dusk they have to come out to feed - and ten million bats have some appetite. The bats in this one cave alone eat 100 tonnes of insects every night. When they start to emerge it's one of the most spectacular sights in the whole natural world.