Hmm total darkness, I have two ways that I know of doing it. The first only works for shorter ranges and involves using the focus assist lamp on the flash. Otherwise manual focus?
You could try using a torch to light up something you want to focus on perhaps. I bet you see some interesting sights down there.
Oh that much darkness! Adam's suggestion is great, painting with light can be very effective. A long exposure with multiple flashes from different locations should give some very speccy results but you will need to experiment with the exposure times and flash power a bit.
I was thinking that the multiple flash idea was a good one. Still have the problem of focusing properly I think I will just has to use a couple of big torches so i can maual focus properly.
Hm... good question... I think most of the AF assist "beams" on compacts would be useless at distances over say 3 metres.
If the subject is in the distance perhaps you could manual focus at infinity, or just estimate the distance and use MF.
Those with infrared beams like on some SLRS or external flashes (like you pointed out) should work quite well...
Many of the "normal" smaller zooms that I have seen don't manual focus to "infinity" unless you can see what you are focusing on. Looks like I will have to pull out my "old" 50mm primecos that does focus at infinity in the proper sense.
Did you ever try a flashlight?
A flashlight will only light a small area, we all have headlights on out helmets of course. I am going caving again weekend after next but I am leading a party of younger people so will not have much time to take many photos for myself, will probably just take my V1 and get a few snaps of them crawling through some of the tighter places.