Updated: 17/10/2011 11:33 | By Charlotte Amelines, contributor, MSN Travel
30 natural marvels you’ve probably never heard of

Bracken Cave, Texas

Bats fly from the Bracken Cave in Texas (Image © Aerie Nature Series/CORBIS)
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It’s hard to imagine 20 million animals living huddled together in one cave little bigger than a football field. But that is exactly what can be found at Bracken Cave, near San Antonio, which contains the biggest bat colony in the world and the largest known concentration of mammals on the planet. At twilight, an immense river of bats – all 20 million of them – stream out of the cave in a column so thick that it shows up on air traffic control.

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