Into the Green: Congaree National Park
It's weird to refer to a National Park as a secret, but it seems that the Congaree National Park is one of South Carolina's best kept secrets. The park features the "largest intact expanse of old growth bottomland hardwood forest remaining in the southeastern United States." I first visited the park, which wasn't designated a National Park until 2003, as part of a environmental engineering class during undergrad, and I've returned twice since. If you haven't been yet, I'd strongly recommend it.