I would be very wary indeed taking autofocus or zoom lenses underground - I've already ruined one autofocus kit lens due to grit ingress, as the repair costs were not much less than buying a new one! I use a Pentax K20D camera, which is very good for allowing old lenses to be used - I mainly use a 28mm prime film lens, which is wide-angle enough for most shots, even with a 1.5X sensor, though a secondhand 21mm would be nice. Prime lenses are only £30 for a total strip-down and clean at my local shop.
I generally use lights and tripod for 'good' shots - I'm considering flash triggers, but the Yongnuo units don't work with Pentax, and my tentative experiments with Firefly triggers are not proving positive - the trigger/flash works, but the photo is totally underexposed, no matter what setting I have the camera on - very perplexing. Being an impatient sod, with a lot of photos to produce soon, I just invested some spare cash in a new underground-friendly tripod instead!