The nation's third-longest cave winds under the hills near Custer, its walls glittering with calcite crystals. South Dakota wears its history on the land itself, and this landmark is a case in point.

Surveyors have mapped more than 200 miles of passages, with more discovered nearly every year.

What sets it apart

The cave takes its name from the calcite spar crystals that line many of its chambers. It is the kind of specific that turns a passing curiosity into a lasting fascination.

It became a national monument in 1908, long before its full extent was understood.

Why it matters

Step back, and a larger picture comes into view: this is South Dakota in miniature, rugged and unhurried. However you come to it, this is a place that stays with you long after the drive home.