Go underground: Exploring Virginia's caverns — usatoday.com

Thanks to its location alongside the Blue Ridge Mountains, Shenandoah Valley sits on a limestone base – the geological equivalent of resting on Swiss cheese. Limestone is the softest type of rock formation in nature, and dissolves in contact with rainwater – about 1/10th of an inch every year. Thus the valley boasts an impressive collection of caves that go for 150 miles. There are seven commercial caves, ranging from quirky mom-n-pop ventures to US National Landmarks.

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