Cool video! It's always a bit of a bummer not finding a good entrance but such is the nature of the beast. Them foundations make for some awesome campsites though, especially where I'm at in California because the ground is covered in sharp rocks. I was out last weekend around Joshua Tree and got to explore some pretty intricate mine sites that are now reduced primarily to foundations and ruined collars. If you ever get a chance to explore the inside of a mine I would highly recommend it if you have the gear and know how (otherwise they can get pretty sketchy, fast).
1. The impromptu campsite.
2.
3.
4. Shorings set up crossing a massively long stope.
5. More foundations.
Since this is actually the video section here is a rough cut of a pretty cool rappel from a few weeks back. I would bet you have some nicer mines in Colorado too.
edit: my bad, forgot link:
http://youtu.be/57GXCm5XVzk