USA - South Dakota - Wind Cave National Park (3)

The cave passages are said to "breathe" because air flows in and out, making the air pressure inside the cave the same as outside. 

Sent by Kimberley from Utah, USA. 

This is my third postcard of Wind Cave National Park. The other two are here and here.

Wind Cave National Park protects two very different worlds - one deep within the earth, the other a sunlit world of many resources. Bison, elk, and other wildlife roam the rolling prairie grasslands and forested hillsides of one of America's oldest national parks. Below the remnant island of intact prairie sits Wind Cave, one of the longest and most complex caves in the world (read more).