Lehman Caves, Great Basin National Park
The Lehman Caves are accessed through the Visitor's Center at the park. (By the way, Great Basin National Park is in central Nevada, very near the Utah border, and east of Ely.)
30, 60, and 90 minutes tours of the caves are available. We took a 60 minute tour as the 90 minute one wasn't available that afternoon. The tour was fascinating. The formations and vast size of the caves is something to be experienced in person. Photos don't do it justice.