In western South Dakota, Wind Cave was established in 1903 and is the first and only park to protect a cave, the sixth largest known cave in the world. The cave has more than 209 km (~130 mi) of explored passages and new passages are said to be discovered at an average rate of 6.4 km (4 mi) per year. The cave’s name comes from the rushing, breathing sound made as air enters and leaves to equal up the pressure inside and out. The fewer cave openings, the more noticeable this is.
Wind Cave is relatively dry inside compared with other caves and so there are few stalactites and stalagmites formed inside. Its most stunning feature is its boxwork, a honeycomb patterning on the walls and ceilings caused when thin blades of the mineral calcite are left behind as erosion occurs. The exact process is still not fully understood, but this is the most extensive display of the phenomenon in the world. Another beautiful sight is the frostwork, delicate crystal formations that look like enlarged snowflakes.
When you’ve seen what the caves have to offer, the park also includes 136 km2 (52.5 mi2) above ground. Here it is a very different story. The terrain is a mixture of 70% mixed grass prairie and 30% ponderosa pine forest and river habitats. The prairie conditions are excellent for supporting bison, reintroduced here in the early 1900s after populations all over America were reduced to near extinction. Elk, bison, pronghorns, wild turkeys and prairie dogs all roam here alongside a host of other small mammals.
In total, there are 48 km (~30 mi) of hiking trails to explore, including three nature trails (all loops, all one mile long).