The Empire Mine State Historic Park is located outside Grass Valley, California & features an outdoor exhibit & trail system surrounding what was once one of the state's largest, oldest, longest, richest, & biggest gold-producing mine.  The Empire Mine was in operation from 1850 to 1956 & produced over eight-billion dollars worth of gold in today's standards.  The mine included over 367 miles of underground tunnels (or nearly the driving distance from San Francisco to Los Angeles, California) to an incline of 11,000 feet w/ a total depth of over a mile below the surface.  Today nearly all of the tunnels are flooded w/ underground water starting at 150 feet below the surface.  Seen here is the main 30 degree-angled inclined shaft from the viewing area looking down showing the rail tracks & manway.  A ride deep into the depths of the mine from this entrance could take as long as half an hour, all under the jurisdiction of the hoist operator in a separate building above.  Amazingly this mine in its 106 years of operation only had 26 deaths.  I used a pole for this shot, I think we all know that if there was any public access I'd be descending this tunnel in a heartbeat.