Concur with the comments on the Go Pro - I have the original HD Hero (the one that doesn't have the improved low light sensitivity)...
It's absolutely fantastic in daylight.
It's great in metal mines, confined spaces or passages in slate mines. Really very good.
But it struggles like the proverbial in big slate chambers unless you can throw thousands of lumens around.
The HD Hero 2 should be better; but even so its my understanding that these little CMOS sensors do not absorb as much light as the CCD sensors that are in most digital cameras.
It is however a fantastic little unit, good underground in small spaces, and should perform OK in large spaces
if you can put enough light in.
But don't expect miracles in big open underground chambers - unless you flood it with light and/or set up multiple light sources you're going to get a video of where the torch beam is pointing in a sort of wierd trippy fear and loathing stylee 😉
my orders are to sit here and watch the world go by