Alas, Denver is not in a cave-rich area, but there are significant caves within a one hour to 90 minute drive from the metropolitan area.
The Cave of the Winds area near Manitou Springs is probably the closest, with over 70 known caves. The Williams Canyon Project of the NSS can provide access to the caves of the canyon for NSS members and friends from out of state. For information regarding the project, visit
http://www.ColoradoCaves.org.
Depending on how quickly winter arrives this October, there also are caves in Colorado's high country, at elevations from 8000 to 11000+ feet. Caves are found south of Eagle, about two hours west of Denver, and north of Glenwood Springs, about three hours west of Denver.
Finally, about 10 minutes west of Golden, there are the Fault Caves in Clear Creek Canyon. These granite fault caves are dusty and filled with cactus spines, but they offer an opportunity to get underground if time is short.