Flash cubes
I haven't seen any of the old cubes underground for years.
To get any sense of depth, in an area where it's naturally profoundly black, you need multiple light sources to produce shadows. For years I (and all my friends) have been using multiple electronic flash guns with Firefly slaves to trigger off the camera's flash.
Being totally in control of all the light in the cave scene is wonderful at times, but also painful if you have the shutter held open and are stumbling around in the dark manually letting off the flash at various points around the cave.