Oregon Caves National Monument

Nestled deep inside the Siskiyou Mountains, the caves formed as rainwater from the ancient forest above dissolved the surrounding marble and created one of the world's few marble caves. The highly complex geology found on the Monument contributes to the unusual and rare plants and animals found nowhere else but here. Experience the extensively decorated and adventurous "Marble Halls of Oregon."

As seen on http://www.nps.gov/orca/index.htm