I know this has been done on here before, but I went through the other day and took a few shots, so thought I'd post em.
( Took a couple of torches and did a bit of 'light painting' - Like a knob, I forgot my remote shutter release!
so for a few shots on the bulb setting, I had to sit there with my finger on the shutter, whilst with the other hand
fanning a big torch around the place, but trying not to shake the camera! Stuck to max 30 second exposure most of the time which to be honest, is well ample enough )
My personal favourite is the second picture (it looks a lot better full-size )
The tunnel isn't straight, and therefore you can't see right through to the other end, so as a result,
it is very dark in there... So don't be fooled by some of pics and think it's quite light inside.
If anyone visits, ( and it's well worth it ), defo take a couple of big torches!
...and... my tip of the day... a little miners-style torch for your head, so you can see the camera controls (it really cuts out a lot of hassles, having your hands free)