The caves are host to a library of Kryptonian information that can only be accessed by placing the octagonal key into its keyhole. Below is a wide shot of the keyhole as it appeared in "Skinwalker" and its original surrounding drawing.
The keyhole changed over the course of many crucial events during "Smallville," and other uses for the octagonal key were revealed; more information is available at the Kryptonian gallery pictures of the keyhole.